Re: [PATCH v2 4/5] scsi: save/load SCSI reservation state

[Stefan Hajnoczi <stefanha@redhat.com>]

[-- Attachment #1: Type: text/plain, Size: 13386 bytes --]

On Mon, Jan 26, 2026 at 07:59:55PM +0000, Daniel P. Berrangé wrote:
> On Mon, Jan 26, 2026 at 02:47:34PM -0500, Stefan Hajnoczi wrote:
> > Add a vmstate subsection to SCSIDiskState so that scsi-block devices can
> > transfer their reservation state during live migration. Upon loading the
> > subsection, the destination QEMU invokes the PERSISTENT RESERVE OUT
> > command's PREEMPT service action to atomically move the reservation from
> > the source I_T nexus to the destination I_T nexus. This results in
> > transparent live migration of SCSI reservations.
> > 
> > This approach is incomplete since SCSI reservations are cooperative and
> > other hosts could interfere. Neither the source QEMU nor the destination
> > QEMU are aware of changes made by other hosts. The assumption is that
> > reservation is not taken over by a third host without cooperation from
> > the source host.
> > 
> > I considered adding the vmstate subsection to SCSIDevice instead of
> > SCSIDiskState, since reservations are part of the SCSI Primary Commands
> > that other devices apart from disks could support. However, due to
> > fragility of migrating reservations, we will probably limit support to
> > scsi-block and maybe scsi-disk in the future. In the end, I think it
> > makes sense to place this within scsi-disk.c.
> > 
> > Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
> > ---
> >  include/hw/scsi/scsi.h |  1 +
> >  hw/core/machine.c      |  4 ++-
> >  hw/scsi/scsi-disk.c    | 81 ++++++++++++++++++++++++++++++++++++++++-
> >  hw/scsi/scsi-generic.c | 82 ++++++++++++++++++++++++++++++++++++++++++
> >  hw/scsi/trace-events   |  1 +
> >  5 files changed, 167 insertions(+), 2 deletions(-)
> > 
> > diff --git a/include/hw/scsi/scsi.h b/include/hw/scsi/scsi.h
> > index c5ec58089b..a3e246dbd9 100644
> > --- a/include/hw/scsi/scsi.h
> > +++ b/include/hw/scsi/scsi.h
> > @@ -253,6 +253,7 @@ SCSIDevice *scsi_device_get(SCSIBus *bus, int channel, int target, int lun);
> >  
> >  /* scsi-generic.c. */
> >  extern const SCSIReqOps scsi_generic_req_ops;
> > +bool scsi_generic_pr_state_preempt(SCSIDevice *s, Error **errp);
> >  
> >  /* scsi-disk.c */
> >  #define SCSI_DISK_QUIRK_MODE_PAGE_APPLE_VENDOR             0
> > diff --git a/hw/core/machine.c b/hw/core/machine.c
> > index 6411e68856..16134f8ce5 100644
> > --- a/hw/core/machine.c
> > +++ b/hw/core/machine.c
> > @@ -38,7 +38,9 @@
> >  #include "hw/acpi/generic_event_device.h"
> >  #include "qemu/audio.h"
> >  
> > -GlobalProperty hw_compat_10_2[] = {};
> > +GlobalProperty hw_compat_10_2[] = {
> > +    { "scsi-block", "migrate-pr", "off" },
> > +};
> >  const size_t hw_compat_10_2_len = G_N_ELEMENTS(hw_compat_10_2);
> >  
> >  GlobalProperty hw_compat_10_1[] = {
> > diff --git a/hw/scsi/scsi-disk.c b/hw/scsi/scsi-disk.c
> > index 76fe5f085b..8845ab1192 100644
> > --- a/hw/scsi/scsi-disk.c
> > +++ b/hw/scsi/scsi-disk.c
> > @@ -28,6 +28,7 @@
> >  #include "qemu/hw-version.h"
> >  #include "qemu/memalign.h"
> >  #include "hw/scsi/scsi.h"
> > +#include "migration/misc.h"
> >  #include "migration/qemu-file-types.h"
> >  #include "migration/vmstate.h"
> >  #include "hw/scsi/emulation.h"
> > @@ -122,6 +123,7 @@ struct SCSIDiskState {
> >       */
> >      uint16_t rotation_rate;
> >      bool migrate_emulated_scsi_request;
> > +    NotifierWithReturn migration_notifier;
> >  };
> >  
> >  static void scsi_free_request(SCSIRequest *req)
> > @@ -2737,6 +2739,25 @@ static SCSIRequest *scsi_new_request(SCSIDevice *d, uint32_t tag, uint32_t lun,
> >  }
> >  
> >  #ifdef __linux__
> > +/*
> > + * Preempt on the SCSI Persistent Reservation on the source when migration
> > + * fails because the destination may have already preempted and we need to get
> > + * the reservation back.
> > + */
> > +static int scsi_block_migration_notifier(NotifierWithReturn *notifier,
> > +                                         MigrationEvent *e, Error **errp)
> > +{
> > +    if (e->type == MIG_EVENT_PRECOPY_FAILED) {
> > +        SCSIDiskState *s =
> > +            container_of(notifier, SCSIDiskState, migration_notifier);
> > +        SCSIDevice *d = &s->qdev;
> > +
> > +        /* MIG_EVENT_PRECOPY_FAILED cannot fail, so ignore errors */
> > +        scsi_generic_pr_state_preempt(d, NULL);
> 
> I feel like we ought to 'warn_report'  any errors related to failing
> to acquire persistent reservations.
> 
> In the unlikely event an error occurs, whomever has to deal with
> the resulting support ticket will want to know something went wrong
> from the QEMU logs.

Good idea.

I'm also not sure how to best approach logging in general. Usually QEMU
does little logging when the VM is running, but it has become
increasingly difficult to get information out of QEMU via tracing or
monitor commands since nowadays QEMU might be running in a locked down
container. Debugging PR migration issues would be easiest if the trace
events introduced in this series were actually printfs to stderr, but
that's not traditionally how QEMU did things.

> 
> 
> > @@ -3209,6 +3240,46 @@ static const Property scsi_hd_properties[] = {
> >      DEFINE_BLOCK_CHS_PROPERTIES(SCSIDiskState, qdev.conf),
> >  };
> >  
> > +#ifdef __linux__
> > +static bool scsi_disk_pr_state_post_load_errp(void *opaque, int version_id, Error **errp)
> > +{
> > +    SCSIDiskState *s = opaque;
> > +    SCSIDevice *dev = &s->qdev;
> > +
> > +    return scsi_generic_pr_state_preempt(dev, errp);
> > +}
> 
> What if there are multiple SCSI disks, and on the target host
> we successful acquire the reservation for the 1st, but fail
> the second ? Are we ignoring the failure of the second, or
> are we discarding the reservation of the 1st and failing
> migration ?   Should we warn_report here too ?

Each disk is its own scsi-block device.
scsi_disk_pr_state_post_load_errp() will be invoked for each such
device. If one of these calls fails, the migration will abort and the
errp here will reported.

The rollback on the source host doesn't care which devices
succeeded/failed, it performs idempotent PREEMPT operations for all
devices so we're sure all reservations are back on the source host after
a failed migration.

> 
> > +
> > +static bool scsi_disk_pr_state_needed(void *opaque)
> > +{
> > +    SCSIDiskState *s = opaque;
> > +    SCSIPRState *pr_state = &s->qdev.pr_state;
> > +    bool ret;
> > +
> > +    if (!s->qdev.migrate_pr) {
> > +        return false;
> > +    }
> > +
> > +    /* A reservation requires a key, so checking this field is enough */
> > +    WITH_QEMU_LOCK_GUARD(&pr_state->mutex) {
> > +        ret = pr_state->key;
> > +    }
> > +    return ret;
> > +}
> > +
> > +static const VMStateDescription vmstate_scsi_disk_pr_state = {
> > +    .name = "scsi-disk/pr",
> > +    .version_id = 1,
> > +    .minimum_version_id = 1,
> > +    .post_load_errp = scsi_disk_pr_state_post_load_errp,
> > +    .needed = scsi_disk_pr_state_needed,
> > +    .fields = (const VMStateField[]) {
> > +        VMSTATE_UINT64(qdev.pr_state.key, SCSIDiskState),
> > +        VMSTATE_UINT8(qdev.pr_state.resv_type, SCSIDiskState),
> > +        VMSTATE_END_OF_LIST()
> > +    }
> > +};
> > +#endif /* __linux__ */
> > +
> >  static const VMStateDescription vmstate_scsi_disk_state = {
> >      .name = "scsi-disk",
> >      .version_id = 1,
> > @@ -3221,7 +3292,13 @@ static const VMStateDescription vmstate_scsi_disk_state = {
> >          VMSTATE_BOOL(tray_open, SCSIDiskState),
> >          VMSTATE_BOOL(tray_locked, SCSIDiskState),
> >          VMSTATE_END_OF_LIST()
> > -    }
> > +    },
> > +    .subsections = (const VMStateDescription * const []) {
> > +#ifdef __linux__
> > +        &vmstate_scsi_disk_pr_state,
> > +#endif
> > +        NULL
> > +    },
> >  };
> >  
> >  static void scsi_hd_class_initfn(ObjectClass *klass, const void *data)
> > @@ -3301,6 +3378,7 @@ static const Property scsi_block_properties[] = {
> >                        -1),
> >      DEFINE_PROP_UINT32("io_timeout", SCSIDiskState, qdev.io_timeout,
> >                         DEFAULT_IO_TIMEOUT),
> > +    DEFINE_PROP_BOOL("migrate-pr", SCSIDiskState, qdev.migrate_pr, true),
> >  };
> >  
> >  static void scsi_block_class_initfn(ObjectClass *klass, const void *data)
> > @@ -3310,6 +3388,7 @@ static void scsi_block_class_initfn(ObjectClass *klass, const void *data)
> >      SCSIDiskClass *sdc = SCSI_DISK_BASE_CLASS(klass);
> >  
> >      sc->realize      = scsi_block_realize;
> > +    sc->unrealize    = scsi_block_unrealize;
> >      sc->alloc_req    = scsi_block_new_request;
> >      sc->parse_cdb    = scsi_block_parse_cdb;
> >      sdc->dma_readv   = scsi_block_dma_readv;
> > diff --git a/hw/scsi/scsi-generic.c b/hw/scsi/scsi-generic.c
> > index 392647e2b2..e44979faeb 100644
> > --- a/hw/scsi/scsi-generic.c
> > +++ b/hw/scsi/scsi-generic.c
> > @@ -419,6 +419,88 @@ static void scsi_handle_persistent_reserve_out_reply(
> >      }
> >  }
> >  
> > +static bool scsi_generic_pr_register(SCSIDevice *s, uint64_t key, Error **errp)
> > +{
> > +    uint8_t cmd[10] = {};
> > +    uint8_t buf[24] = {};
> > +    uint64_t key_be = cpu_to_be64(key);
> > +    int ret;
> > +
> > +    cmd[0] = PERSISTENT_RESERVE_OUT;
> > +    cmd[1] = PRO_REGISTER;
> > +    cmd[8] = sizeof(buf);
> > +    memcpy(&buf[8], &key_be, sizeof(key_be));
> > +
> > +    ret = scsi_SG_IO(s->conf.blk, SG_DXFER_TO_DEV, cmd, sizeof(cmd),
> > +                     buf, sizeof(buf), s->io_timeout, errp);
> > +    if (ret < 0) {
> > +        error_prepend(errp, "PERSISTENT RESERVE OUT with REGISTER");
> > +        return false;
> > +    }
> > +    return true;
> > +}
> > +
> > +static bool scsi_generic_pr_preempt(SCSIDevice *s, uint64_t key, uint8_t resv_type, Error **errp)
> > +{
> > +    uint8_t cmd[10] = {};
> > +    uint8_t buf[24] = {};
> > +    uint64_t key_be = cpu_to_be64(key);
> > +    int ret;
> > +
> > +    cmd[0] = PERSISTENT_RESERVE_OUT;
> > +    cmd[1] = PRO_PREEMPT;
> > +    cmd[2] = resv_type & 0xf;
> > +    cmd[8] = sizeof(buf);
> > +    memcpy(&buf[0], &key_be, sizeof(key_be));
> > +    memcpy(&buf[8], &key_be, sizeof(key_be));
> > +
> > +    ret = scsi_SG_IO(s->conf.blk, SG_DXFER_TO_DEV, cmd, sizeof(cmd),
> > +                     buf, sizeof(buf), s->io_timeout, errp);
> > +    if (ret < 0) {
> > +        error_prepend(errp, "PERSISTENT RESERVE OUT with PREEMPT");
> > +        return false;
> > +    }
> > +    return true;
> > +}
> > +
> > +/* Register keys and preempt reservations after live migration */
> > +bool scsi_generic_pr_state_preempt(SCSIDevice *s, Error **errp)
> > +{
> > +    SCSIPRState *pr_state = &s->pr_state;
> > +    uint64_t key;
> > +    uint8_t resv_type;
> > +
> > +    WITH_QEMU_LOCK_GUARD(&pr_state->mutex) {
> > +        key = pr_state->key;
> > +        resv_type = pr_state->resv_type;
> > +    }
> > +
> > +    trace_scsi_generic_pr_state_preempt(key, resv_type);
> > +
> > +    if (key) {
> > +        if (!scsi_generic_pr_register(s, key, errp)) {
> > +            return false;
> > +        }
> > +
> > +        /*
> > +         * Two cases:
> > +         *
> > +         * 1. There is no reservation (resv_type is 0) and the other I_T nexus
> > +         *    will be unregistered. This is important so the source host does
> > +         *    not leak registered keys across live migration.
> > +         *
> > +         * 2. There is a reservation (resv_type is not 0) and the other I_T
> > +         *    nexus will be unregistered and its reservation is atomically
> > +         *    taken over by us. This is the scenario where a reservation is
> > +         *    migrated along with the guest.
> > +         */
> > +        if (!scsi_generic_pr_preempt(s, key, resv_type, errp)) {
> > +            return false;
> > +        }
> > +    }
> > +    return true;
> > +}
> > +
> >  static void scsi_read_complete(void * opaque, int ret)
> >  {
> >      SCSIGenericReq *r = (SCSIGenericReq *)opaque;
> > diff --git a/hw/scsi/trace-events b/hw/scsi/trace-events
> > index ff92fff7c5..a8ac1e7f1d 100644
> > --- a/hw/scsi/trace-events
> > +++ b/hw/scsi/trace-events
> > @@ -391,3 +391,4 @@ scsi_generic_aio_sgio_command(uint32_t tag, uint8_t cmd, uint32_t timeout) "gene
> >  scsi_generic_ioctl_sgio_command(uint8_t cmd, uint32_t timeout) "generic ioctl sgio: cmd=0x%x timeout=%u"
> >  scsi_generic_ioctl_sgio_done(uint8_t cmd, int ret, uint8_t status, uint8_t host_status) "generic ioctl sgio: cmd=0x%x ret=%d status=0x%x host_status=0x%x"
> >  scsi_generic_persistent_reserve_out_reply(uint8_t service_action, uint8_t resv_type, uint64_t old_key, uint64_t new_key) "persistent reserve out reply service_action=%u resv_type=%u old_key=0x%" PRIx64 " new_key=0x%" PRIx64
> > +scsi_generic_pr_state_preempt(uint64_t key, uint8_t resv_type) "key=0x%" PRIx64 " resv_type=%u"
> > -- 
> > 2.52.0
> > 
> > 
> 
> With regards,
> Daniel
> -- 
> |: https://berrange.com      -o-    https://www.flickr.com/photos/dberrange :|
> |: https://libvirt.org         -o-            https://fstop138.berrange.com :|
> |: https://entangle-photo.org    -o-    https://www.instagram.com/dberrange :|
> 

[-- Attachment #2: signature.asc --]
[-- Type: application/pgp-signature, Size: 488 bytes --]