hyperv.c

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/*
 * Copyright (C) 2014 Michael Brown <mbrown@fensystems.co.uk>.
 *
 * This program is free software; you can redistribute it and/or
 * modify it under the terms of the GNU General Public License as
 * published by the Free Software Foundation; either version 2 of the
 * License, or (at your option) any later version.
 *
 * This program is distributed in the hope that it will be useful, but
 * WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
 * General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License
 * along with this program; if not, write to the Free Software
 * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA
 * 02110-1301, USA.
 *
 * You can also choose to distribute this program under the terms of
 * the Unmodified Binary Distribution Licence (as given in the file
 * COPYING.UBDL), provided that you have satisfied its requirements.
 */
 
FILE_LICENCE ( GPL2_OR_LATER_OR_UBDL );
 
/** @file
 *
 * Hyper-V driver
 *
 */
 
#include <stdlib.h>
#include <stdarg.h>
#include <string.h>
#include <unistd.h>
#include <assert.h>
#include <errno.h>
#include <byteswap.h>
#include <pic8259.h>
#include <ipxe/malloc.h>
#include <ipxe/device.h>
#include <ipxe/timer.h>
#include <ipxe/quiesce.h>
#include <ipxe/cpuid.h>
#include <ipxe/msr.h>
#include <ipxe/hyperv.h>
#include <ipxe/vmbus.h>
#include "hyperv.h"
 
/** Maximum time to wait for a message response
 *
 * This is a policy decision.
 */
#define HV_MESSAGE_MAX_WAIT_MS 1000
 
/** Hyper-V timer frequency (fixed 10Mhz) */
#define HV_TIMER_HZ 10000000
 
/** Hyper-V timer scale factor (used to avoid 64-bit division) */
#define HV_TIMER_SHIFT 18
 
/**
 * Convert a Hyper-V status code to an iPXE status code
 *
 * @v status            Hyper-V status code
 * @ret rc              iPXE status code (before negation)
 */
#define EHV( status ) EPLATFORM ( EINFO_EPLATFORM, (status) )
 
/**
 * Allocate zeroed pages
 *
 * @v hv                Hyper-V hypervisor
 * @v ...               Page addresses to fill in, terminated by NULL
 * @ret rc              Return status code
 */
__attribute__ (( sentinel )) int
hv_alloc_pages ( struct hv_hypervisor *hv, ... ) {
        va_list args;
        void **page;
        int i;
 
        /* Allocate and zero pages */
        va_start ( args, hv );
        for ( i = 0 ; ( ( page = va_arg ( args, void ** ) ) != NULL ); i++ ) {
                *page = malloc_phys ( PAGE_SIZE, PAGE_SIZE );
                if ( ! *page )
                        goto err_alloc;
                memset ( *page, 0, PAGE_SIZE );
        }
        va_end ( args );
 
        return 0;
 
 err_alloc:
        va_end ( args );
        va_start ( args, hv );
        for ( ; i >= 0 ; i-- ) {
                page = va_arg ( args, void ** );
                free_phys ( *page, PAGE_SIZE );
        }
        va_end ( args );
        return -ENOMEM;
}
 
/**
 * Free pages
 *
 * @v hv                Hyper-V hypervisor
 * @v ...               Page addresses, terminated by NULL
 */
__attribute__ (( sentinel )) void
hv_free_pages ( struct hv_hypervisor *hv, ... ) {
        va_list args;
        void *page;
 
        va_start ( args, hv );
        while ( ( page = va_arg ( args, void * ) ) != NULL )
                free_phys ( page, PAGE_SIZE );
        va_end ( args );
}
 
/**
 * Allocate message buffer
 *
 * @v hv                Hyper-V hypervisor
 * @ret rc              Return status code
 */
static int hv_alloc_message ( struct hv_hypervisor *hv ) {
 
        /* Allocate buffer.  Must be aligned to at least 8 bytes and
         * must not cross a page boundary, so align on its own size.
         */
        hv->message = malloc_phys ( sizeof ( *hv->message ),
                                    sizeof ( *hv->message ) );
        if ( ! hv->message )
                return -ENOMEM;
 
        return 0;
}
 
/**
 * Free message buffer
 *
 * @v hv                Hyper-V hypervisor
 */
static void hv_free_message ( struct hv_hypervisor *hv ) {
 
        /* Free buffer */
        free_phys ( hv->message, sizeof ( *hv->message ) );
}
 
/**
 * Check whether or not we are running in Hyper-V
 *
 * @ret rc              Return status code
 */
static int hv_check_hv ( void ) {
        struct x86_features features;
        uint32_t interface_id;
        uint32_t discard_ebx;
        uint32_t discard_ecx;
        uint32_t discard_edx;
 
        /* Check for presence of a hypervisor (not necessarily Hyper-V) */
        x86_features ( &features );
        if ( ! ( features.intel.ecx & CPUID_FEATURES_INTEL_ECX_HYPERVISOR ) ) {
                DBGC ( HV_INTERFACE_ID, "HV not running in a hypervisor\n" );
                return -ENODEV;
        }
 
        /* Check that hypervisor is Hyper-V */
        cpuid ( HV_CPUID_INTERFACE_ID, 0, &interface_id, &discard_ebx,
                &discard_ecx, &discard_edx );
        if ( interface_id != HV_INTERFACE_ID ) {
                DBGC ( HV_INTERFACE_ID, "HV not running in Hyper-V (interface "
                       "ID %#08x)\n", interface_id );
                return -ENODEV;
        }
 
        return 0;
}
 
/**
 * Check required features
 *
 * @v hv                Hyper-V hypervisor
 * @ret rc              Return status code
 */
static int hv_check_features ( struct hv_hypervisor *hv ) {
        uint32_t available;
        uint32_t permissions;
        uint32_t discard_ecx;
        uint32_t discard_edx;
 
        /* Check that required features and privileges are available */
        cpuid ( HV_CPUID_FEATURES, 0, &available, &permissions, &discard_ecx,
                &discard_edx );
        if ( ! ( available & HV_FEATURES_AVAIL_HYPERCALL_MSR ) ) {
                DBGC ( hv, "HV %p has no hypercall MSRs (features %08x:%08x)\n",
                       hv, available, permissions );
                return -ENODEV;
        }
        if ( ! ( available & HV_FEATURES_AVAIL_SYNIC_MSR ) ) {
                DBGC ( hv, "HV %p has no SynIC MSRs (features %08x:%08x)\n",
                       hv, available, permissions );
                return -ENODEV;
        }
        if ( ! ( permissions & HV_FEATURES_PERM_POST_MESSAGES ) ) {
                DBGC ( hv, "HV %p cannot post messages (features %08x:%08x)\n",
                       hv, available, permissions );
                return -EACCES;
        }
        if ( ! ( permissions & HV_FEATURES_PERM_SIGNAL_EVENTS ) ) {
                DBGC ( hv, "HV %p cannot signal events (features %08x:%08x)",
                       hv, available, permissions );
                return -EACCES;
        }
 
        return 0;
}
 
/**
 * Check that Gen 2 UEFI firmware is not running
 *
 * @v hv                Hyper-V hypervisor
 * @ret rc              Return status code
 *
 * We must not steal ownership from the Gen 2 UEFI firmware, since
 * doing so will cause an immediate crash.  Avoid this by checking for
 * the guest OS identity known to be used by the Gen 2 UEFI firmware.
 */
static int hv_check_uefi ( struct hv_hypervisor *hv ) {
        uint64_t guest_os_id;
 
        /* Check for UEFI firmware's guest OS identity */
        guest_os_id = rdmsr ( HV_X64_MSR_GUEST_OS_ID );
        if ( guest_os_id == HV_GUEST_OS_ID_UEFI ) {
                DBGC ( hv, "HV %p is owned by UEFI firmware\n", hv );
                return -ENOTSUP;
        }
 
        return 0;
}
 
/**
 * Map hypercall page
 *
 * @v hv                Hyper-V hypervisor
 */
static void hv_map_hypercall ( struct hv_hypervisor *hv ) {
        union {
                struct {
                        uint32_t ebx;
                        uint32_t ecx;
                        uint32_t edx;
                } __attribute__ (( packed ));
                char text[ 13 /* "bbbbccccdddd" + NUL */ ];
        } vendor_id;
        uint32_t build;
        uint32_t version;
        uint32_t discard_eax;
        uint32_t discard_ecx;
        uint32_t discard_edx;
        uint64_t guest_os_id;
        uint64_t hypercall;
 
        /* Report guest OS identity */
        guest_os_id = rdmsr ( HV_X64_MSR_GUEST_OS_ID );
        if ( guest_os_id != 0 ) {
                DBGC ( hv, "HV %p guest OS ID MSR was %#08llx\n",
                       hv, guest_os_id );
        }
        guest_os_id = HV_GUEST_OS_ID_IPXE;
        DBGC2 ( hv, "HV %p guest OS ID MSR is %#08llx\n", hv, guest_os_id );
        wrmsr ( HV_X64_MSR_GUEST_OS_ID, guest_os_id );
 
        /* Get hypervisor system identity (for debugging) */
        cpuid ( HV_CPUID_VENDOR_ID, 0, &discard_eax, &vendor_id.ebx,
                &vendor_id.ecx, &vendor_id.edx );
        vendor_id.text[ sizeof ( vendor_id.text ) - 1 ] = '\0';
        cpuid ( HV_CPUID_HYPERVISOR_ID, 0, &build, &version, &discard_ecx,
                &discard_edx );
        DBGC ( hv, "HV %p detected \"%s\" version %d.%d build %d\n", hv,
               vendor_id.text, ( version >> 16 ), ( version & 0xffff ), build );
 
        /* Map hypercall page */
        hypercall = rdmsr ( HV_X64_MSR_HYPERCALL );
        hypercall &= ( PAGE_SIZE - 1 );
        hypercall |= ( virt_to_phys ( hv->hypercall ) | HV_HYPERCALL_ENABLE );
        DBGC2 ( hv, "HV %p hypercall MSR is %#08llx\n", hv, hypercall );
        wrmsr ( HV_X64_MSR_HYPERCALL, hypercall );
}
 
/**
 * Unmap hypercall page
 *
 * @v hv                Hyper-V hypervisor
 */
static void hv_unmap_hypercall ( struct hv_hypervisor *hv ) {
        uint64_t hypercall;
        uint64_t guest_os_id;
 
        /* Unmap the hypercall page */
        hypercall = rdmsr ( HV_X64_MSR_HYPERCALL );
        hypercall &= ( ( PAGE_SIZE - 1 ) & ~HV_HYPERCALL_ENABLE );
        DBGC2 ( hv, "HV %p hypercall MSR is %#08llx\n", hv, hypercall );
        wrmsr ( HV_X64_MSR_HYPERCALL, hypercall );
 
        /* Reset the guest OS identity */
        guest_os_id = 0;
        DBGC2 ( hv, "HV %p guest OS ID MSR is %#08llx\n", hv, guest_os_id );
        wrmsr ( HV_X64_MSR_GUEST_OS_ID, guest_os_id );
}
 
/**
 * Map synthetic interrupt controller
 *
 * @v hv                Hyper-V hypervisor
 */
static void hv_map_synic ( struct hv_hypervisor *hv ) {
        uint64_t simp;
        uint64_t siefp;
        uint64_t scontrol;
 
        /* Zero SynIC message and event pages */
        memset ( hv->synic.message, 0, PAGE_SIZE );
        memset ( hv->synic.event, 0, PAGE_SIZE );
 
        /* Map SynIC message page */
        simp = rdmsr ( HV_X64_MSR_SIMP );
        simp &= ( PAGE_SIZE - 1 );
        simp |= ( virt_to_phys ( hv->synic.message ) | HV_SIMP_ENABLE );
        DBGC2 ( hv, "HV %p SIMP MSR is %#08llx\n", hv, simp );
        wrmsr ( HV_X64_MSR_SIMP, simp );
 
        /* Map SynIC event page */
        siefp = rdmsr ( HV_X64_MSR_SIEFP );
        siefp &= ( PAGE_SIZE - 1 );
        siefp |= ( virt_to_phys ( hv->synic.event ) | HV_SIEFP_ENABLE );
        DBGC2 ( hv, "HV %p SIEFP MSR is %#08llx\n", hv, siefp );
        wrmsr ( HV_X64_MSR_SIEFP, siefp );
 
        /* Enable SynIC */
        scontrol = rdmsr ( HV_X64_MSR_SCONTROL );
        scontrol |= HV_SCONTROL_ENABLE;
        DBGC2 ( hv, "HV %p SCONTROL MSR is %#08llx\n", hv, scontrol );
        wrmsr ( HV_X64_MSR_SCONTROL, scontrol );
}
 
/**
 * Unmap synthetic interrupt controller, leaving SCONTROL untouched
 *
 * @v hv                Hyper-V hypervisor
 */
static void hv_unmap_synic_no_scontrol ( struct hv_hypervisor *hv ) {
        uint64_t siefp;
        uint64_t simp;
 
        /* Unmap SynIC event page */
        siefp = rdmsr ( HV_X64_MSR_SIEFP );
        siefp &= ( ( PAGE_SIZE - 1 ) & ~HV_SIEFP_ENABLE );
        DBGC2 ( hv, "HV %p SIEFP MSR is %#08llx\n", hv, siefp );
        wrmsr ( HV_X64_MSR_SIEFP, siefp );
 
        /* Unmap SynIC message page */
        simp = rdmsr ( HV_X64_MSR_SIMP );
        simp &= ( ( PAGE_SIZE - 1 ) & ~HV_SIMP_ENABLE );
        DBGC2 ( hv, "HV %p SIMP MSR is %#08llx\n", hv, simp );
        wrmsr ( HV_X64_MSR_SIMP, simp );
}
 
/**
 * Unmap synthetic interrupt controller
 *
 * @v hv                Hyper-V hypervisor
 */
static void hv_unmap_synic ( struct hv_hypervisor *hv ) {
        uint64_t scontrol;
 
        /* Disable SynIC */
        scontrol = rdmsr ( HV_X64_MSR_SCONTROL );
        scontrol &= ~HV_SCONTROL_ENABLE;
        DBGC2 ( hv, "HV %p SCONTROL MSR is %#08llx\n", hv, scontrol );
        wrmsr ( HV_X64_MSR_SCONTROL, scontrol );
 
        /* Unmap SynIC event and message pages */
        hv_unmap_synic_no_scontrol ( hv );
}
 
/**
 * Enable synthetic interrupt
 *
 * @v hv                Hyper-V hypervisor
 * @v sintx             Synthetic interrupt number
 */
void hv_enable_sint ( struct hv_hypervisor *hv, unsigned int sintx ) {
        unsigned long msr = HV_X64_MSR_SINT ( sintx );
        uint64_t sint;
 
        /* Enable synthetic interrupt
         *
         * We have to enable the interrupt, otherwise messages will
         * not be delivered (even though the documentation implies
         * that polling for messages is possible).  We enable AutoEOI
         * and hook the interrupt to the obsolete IRQ13 (FPU
         * exception) vector, which will be implemented as a no-op.
         */
        sint = rdmsr ( msr );
        sint &= ~( HV_SINT_MASKED | HV_SINT_VECTOR_MASK );
        sint |= ( HV_SINT_AUTO_EOI |
                  HV_SINT_VECTOR ( IRQ_INT ( 13 /* See comment above */ ) ) );
        DBGC2 ( hv, "HV %p SINT%d MSR is %#08llx\n", hv, sintx, sint );
        wrmsr ( msr, sint );
}
 
/**
 * Disable synthetic interrupt
 *
 * @v hv                Hyper-V hypervisor
 * @v sintx             Synthetic interrupt number
 */
void hv_disable_sint ( struct hv_hypervisor *hv, unsigned int sintx ) {
        unsigned long msr = HV_X64_MSR_SINT ( sintx );
        uint64_t sint;
 
        /* Do nothing if interrupt is already disabled */
        sint = rdmsr ( msr );
        if ( sint & HV_SINT_MASKED )
                return;
 
        /* Disable synthetic interrupt */
        sint &= ~HV_SINT_AUTO_EOI;
        sint |= HV_SINT_MASKED;
        DBGC2 ( hv, "HV %p SINT%d MSR is %#08llx\n", hv, sintx, sint );
        wrmsr ( msr, sint );
}
 
/**
 * Post message
 *
 * @v hv                Hyper-V hypervisor
 * @v id                Connection ID
 * @v type              Message type
 * @v data              Message
 * @v len               Length of message
 * @ret rc              Return status code
 */
int hv_post_message ( struct hv_hypervisor *hv, unsigned int id,
                      unsigned int type, const void *data, size_t len ) {
        struct hv_post_message *msg = &hv->message->posted;
        int status;
        int rc;
 
        /* Sanity check */
        assert ( len <= sizeof ( msg->data ) );
 
        /* Construct message */
        memset ( msg, 0, sizeof ( *msg ) );
        msg->id = cpu_to_le32 ( id );
        msg->type = cpu_to_le32 ( type );
        msg->len = cpu_to_le32 ( len );
        memcpy ( msg->data, data, len );
        DBGC2 ( hv, "HV %p connection %d posting message type %#08x:\n",
                hv, id, type );
        DBGC2_HDA ( hv, 0, msg->data, len );
 
        /* Post message */
        if ( ( status = hv_call ( hv, HV_POST_MESSAGE, msg, NULL ) ) != 0 ) {
                rc = -EHV ( status );
                DBGC ( hv, "HV %p could not post message to %#08x: %s\n",
                       hv, id, strerror ( rc ) );
                return rc;
        }
 
        return 0;
}
 
/**
 * Wait for received message
 *
 * @v hv                Hyper-V hypervisor
 * @v sintx             Synthetic interrupt number
 * @ret rc              Return status code
 */
int hv_wait_for_message ( struct hv_hypervisor *hv, unsigned int sintx ) {
        struct hv_message *msg = &hv->message->received;
        struct hv_message *src = &hv->synic.message[sintx];
        unsigned int retries;
        size_t len;
 
        /* Wait for message to arrive */
        for ( retries = 0 ; retries < HV_MESSAGE_MAX_WAIT_MS ; retries++ ) {
 
                /* Check for message */
                if ( src->type ) {
 
                        /* Copy message */
                        memset ( msg, 0, sizeof ( *msg ) );
                        len = src->len;
                        assert ( len <= sizeof ( *msg ) );
                        memcpy ( msg, src,
                                 ( offsetof ( typeof ( *msg ), data ) + len ) );
                        DBGC2 ( hv, "HV %p SINT%d received message type "
                                "%#08x:\n", hv, sintx,
                                le32_to_cpu ( msg->type ) );
                        DBGC2_HDA ( hv, 0, msg->data, len );
 
                        /* Consume message */
                        src->type = 0;
 
                        return 0;
                }
 
                /* Trigger message delivery */
                wrmsr ( HV_X64_MSR_EOM, 0 );
 
                /* Delay */
                mdelay ( 1 );
        }
 
        DBGC ( hv, "HV %p SINT%d timed out waiting for message\n",
               hv, sintx );
        return -ETIMEDOUT;
}
 
/**
 * Signal event
 *
 * @v hv                Hyper-V hypervisor
 * @v id                Connection ID
 * @v flag              Flag number
 * @ret rc              Return status code
 */
int hv_signal_event ( struct hv_hypervisor *hv, unsigned int id,
                      unsigned int flag ) {
        struct hv_signal_event *event = &hv->message->signalled;
        int status;
        int rc;
 
        /* Construct event */
        memset ( event, 0, sizeof ( *event ) );
        event->id = cpu_to_le32 ( id );
        event->flag = cpu_to_le16 ( flag );
 
        /* Signal event */
        if ( ( status = hv_call ( hv, HV_SIGNAL_EVENT, event, NULL ) ) != 0 ) {
                rc = -EHV ( status );
                DBGC ( hv, "HV %p could not signal event to %#08x: %s\n",
                       hv, id, strerror ( rc ) );
                return rc;
        }
 
        return 0;
}
 
/**
 * Probe root device
 *
 * @v rootdev           Root device
 * @ret rc              Return status code
 */
static int hv_probe ( struct root_device *rootdev ) {
        struct hv_hypervisor *hv;
        int rc;
 
        /* Check we are running in Hyper-V */
        if ( ( rc = hv_check_hv() ) != 0 )
                goto err_check_hv;
 
        /* Allocate and initialise structure */
        hv = zalloc ( sizeof ( *hv ) );
        if ( ! hv ) {
                rc = -ENOMEM;
                goto err_alloc;
        }
 
        /* Check features */
        if ( ( rc = hv_check_features ( hv ) ) != 0 )
                goto err_check_features;
 
        /* Check that Gen 2 UEFI firmware is not running */
        if ( ( rc = hv_check_uefi ( hv ) ) != 0 )
                goto err_check_uefi;
 
        /* Allocate pages */
        if ( ( rc = hv_alloc_pages ( hv, &hv->hypercall, &hv->synic.message,
                                     &hv->synic.event, NULL ) ) != 0 )
                goto err_alloc_pages;
 
        /* Allocate message buffer */
        if ( ( rc = hv_alloc_message ( hv ) ) != 0 )
                goto err_alloc_message;
 
        /* Map hypercall page */
        hv_map_hypercall ( hv );
 
        /* Map synthetic interrupt controller */
        hv_map_synic ( hv );
 
        /* Probe Hyper-V devices */
        if ( ( rc = vmbus_probe ( hv, &rootdev->dev ) ) != 0 )
                goto err_vmbus_probe;
 
        rootdev_set_drvdata ( rootdev, hv );
        return 0;
 
        vmbus_remove ( hv, &rootdev->dev );
 err_vmbus_probe:
        hv_unmap_synic ( hv );
        hv_unmap_hypercall ( hv );
        hv_free_message ( hv );
 err_alloc_message:
        hv_free_pages ( hv, hv->hypercall, hv->synic.message, hv->synic.event,
                        NULL );
 err_alloc_pages:
 err_check_uefi:
 err_check_features:
        free ( hv );
 err_alloc:
 err_check_hv:
        return rc;
}
 
/**
 * Remove root device
 *
 * @v rootdev           Root device
 */
static void hv_remove ( struct root_device *rootdev ) {
        struct hv_hypervisor *hv = rootdev_get_drvdata ( rootdev );
 
        vmbus_remove ( hv, &rootdev->dev );
        hv_unmap_synic ( hv );
        hv_unmap_hypercall ( hv );
        hv_free_message ( hv );
        hv_free_pages ( hv, hv->hypercall, hv->synic.message, hv->synic.event,
                        NULL );
        free ( hv );
        rootdev_set_drvdata ( rootdev, NULL );
}
 
/** Hyper-V root device driver */
static struct root_driver hv_root_driver = {
        .probe = hv_probe,
        .remove = hv_remove,
};
 
/** Hyper-V root device */
struct root_device hv_root_device __root_device = {
        .dev = { .name = "Hyper-V" },
        .driver = &hv_root_driver,
};
 
/**
 * Quiesce system
 *
 */
static void hv_quiesce ( void ) {
        struct hv_hypervisor *hv = rootdev_get_drvdata ( &hv_root_device );
        unsigned int i;
 
        /* Do nothing if we are not running in Hyper-V */
        if ( ! hv )
                return;
 
        /* The "enlightened" portions of the Windows Server 2016 boot
         * process will not cleanly take ownership of an active
         * Hyper-V connection.  Experimentation shows that the minimum
         * requirement is that we disable the SynIC message page
         * (i.e. zero the SIMP MSR).
         *
         * We cannot perform a full shutdown of the Hyper-V
         * connection.  Experimentation shows that if we disable the
         * SynIC (i.e. zero the SCONTROL MSR) then Windows Server 2016
         * will enter an indefinite wait loop.
         *
         * Attempt to create a safe handover environment by resetting
         * all MSRs except for SCONTROL.
         *
         * Note that we do not shut down our VMBus devices, since we
         * may need to unquiesce the system and continue operation.
         */
 
        /* Disable all synthetic interrupts */
        for ( i = 0 ; i <= HV_SINT_MAX ; i++ )
                hv_disable_sint ( hv, i );
 
        /* Unmap synthetic interrupt controller, leaving SCONTROL
         * enabled (see above).
         */
        hv_unmap_synic_no_scontrol ( hv );
 
        /* Unmap hypercall page */
        hv_unmap_hypercall ( hv );
 
        DBGC ( hv, "HV %p quiesced\n", hv );
}
 
/**
 * Unquiesce system
 *
 */
static void hv_unquiesce ( void ) {
        struct hv_hypervisor *hv = rootdev_get_drvdata ( &hv_root_device );
        uint64_t simp;
        int rc;
 
        /* Do nothing if we are not running in Hyper-V */
        if ( ! hv )
                return;
 
        /* Experimentation shows that the "enlightened" portions of
         * Windows Server 2016 will break our Hyper-V connection at
         * some point during a SAN boot.  Surprisingly it does not
         * change the guest OS ID MSR, but it does leave the SynIC
         * message page disabled.
         *
         * Our own explicit quiescing procedure will also disable the
         * SynIC message page.  We can therefore use the SynIC message
         * page enable bit as a heuristic to determine when we need to
         * reestablish our Hyper-V connection.
         */
        simp = rdmsr ( HV_X64_MSR_SIMP );
        if ( simp & HV_SIMP_ENABLE )
                return;
 
        /* Remap hypercall page */
        hv_map_hypercall ( hv );
 
        /* Remap synthetic interrupt controller */
        hv_map_synic ( hv );
 
        /* Reset Hyper-V devices */
        if ( ( rc = vmbus_reset ( hv, &hv_root_device.dev ) ) != 0 ) {
                DBGC ( hv, "HV %p could not unquiesce: %s\n",
                       hv, strerror ( rc ) );
                /* Nothing we can do */
                return;
        }
}
 
/** Hyper-V quiescer */
struct quiescer hv_quiescer __quiescer = {
        .quiesce = hv_quiesce,
        .unquiesce = hv_unquiesce,
};
 
/**
 * Probe timer
 *
 * @ret rc              Return status code
 */
static int hv_timer_probe ( void ) {
        uint32_t available;
        uint32_t discard_ebx;
        uint32_t discard_ecx;
        uint32_t discard_edx;
        int rc;
 
        /* Check we are running in Hyper-V */
        if ( ( rc = hv_check_hv() ) != 0 )
                return rc;
 
        /* Check for available reference counter */
        cpuid ( HV_CPUID_FEATURES, 0, &available, &discard_ebx, &discard_ecx,
                &discard_edx );
        if ( ! ( available & HV_FEATURES_AVAIL_TIME_REF_COUNT_MSR ) ) {
                DBGC ( HV_INTERFACE_ID, "HV has no time reference counter\n" );
                return -ENODEV;
        }
 
        return 0;
}
 
/**
 * Get current system time in ticks
 *
 * @ret ticks           Current time, in ticks
 */
static unsigned long hv_currticks ( void ) {
 
        /* Calculate time using a combination of bit shifts and
         * multiplication (to avoid a 64-bit division).
         */
        return ( ( rdmsr ( HV_X64_MSR_TIME_REF_COUNT ) >> HV_TIMER_SHIFT ) *
                 ( TICKS_PER_SEC / ( HV_TIMER_HZ >> HV_TIMER_SHIFT ) ) );
}
 
/**
 * Delay for a fixed number of microseconds
 *
 * @v usecs             Number of microseconds for which to delay
 */
static void hv_udelay ( unsigned long usecs ) {
        uint32_t start;
        uint32_t elapsed;
        uint32_t threshold;
 
        /* Spin until specified number of 10MHz ticks have elapsed */
        start = rdmsr ( HV_X64_MSR_TIME_REF_COUNT );
        threshold = ( usecs * ( HV_TIMER_HZ / 1000000 ) );
        do {
                elapsed = ( rdmsr ( HV_X64_MSR_TIME_REF_COUNT ) - start );
        } while ( elapsed < threshold );
}
 
/** Hyper-V timer */
struct timer hv_timer __timer ( TIMER_PREFERRED ) = {
        .name = "Hyper-V",
        .probe = hv_timer_probe,
        .currticks = hv_currticks,
        .udelay = hv_udelay,
};
 
/* Drag in objects via hv_root_device */
REQUIRING_SYMBOL ( hv_root_device );
 
/* Drag in netvsc driver */
REQUIRE_OBJECT ( netvsc );