Re: [PATCH v26 07/17] vfio: Register SaveVMHandlers for VFIO device

From: Alex Williamson <alex.williamson@redhat.com>
To: Kirti Wankhede <kwankhede@nvidia.com>
Cc: cohuck@redhat.com, cjia@nvidia.com, aik@ozlabs.ru,
	Zhengxiao.zx@Alibaba-inc.com, shuangtai.tst@alibaba-inc.com,
	qemu-devel@nongnu.org, peterx@redhat.com, eauger@redhat.com,
	yi.l.liu@intel.com, quintela@redhat.com, ziye.yang@intel.com,
	armbru@redhat.com, mlevitsk@redhat.com, pasic@linux.ibm.com,
	felipe@nutanix.com, zhi.a.wang@intel.com, kevin.tian@intel.com,
	yan.y.zhao@intel.com, dgilbert@redhat.com,
	changpeng.liu@intel.com, eskultet@redhat.com, Ken.Xue@amd.com,
	jonathan.davies@nutanix.com, pbonzini@redhat.com
Subject: Re: [PATCH v26 07/17] vfio: Register SaveVMHandlers for VFIO device
Date: Fri, 25 Sep 2020 14:20:46 -0600	[thread overview]
Message-ID: <20200925142046.7c9eca89@x1.home> (raw)
In-Reply-To: <1600817059-26721-8-git-send-email-kwankhede@nvidia.com>

On Wed, 23 Sep 2020 04:54:09 +0530
Kirti Wankhede <kwankhede@nvidia.com> wrote:

> Define flags to be used as delimeter in migration file stream.
> Added .save_setup and .save_cleanup functions. Mapped & unmapped migration
> region from these functions at source during saving or pre-copy phase.
> Set VFIO device state depending on VM's state. During live migration, VM is
> running when .save_setup is called, _SAVING | _RUNNING state is set for VFIO
> device. During save-restore, VM is paused, _SAVING state is set for VFIO device.
> 
> Signed-off-by: Kirti Wankhede <kwankhede@nvidia.com>
> Reviewed-by: Neo Jia <cjia@nvidia.com>
> ---
>  hw/vfio/migration.c  | 91 ++++++++++++++++++++++++++++++++++++++++++++++++++++
>  hw/vfio/trace-events |  2 ++
>  2 files changed, 93 insertions(+)
> 
> diff --git a/hw/vfio/migration.c b/hw/vfio/migration.c
> index f650fe9fc3c8..8e8adaa25779 100644
> --- a/hw/vfio/migration.c
> +++ b/hw/vfio/migration.c
> @@ -8,12 +8,15 @@
>   */
>  
>  #include "qemu/osdep.h"
> +#include "qemu/main-loop.h"
> +#include "qemu/cutils.h"
>  #include <linux/vfio.h>
>  
>  #include "sysemu/runstate.h"
>  #include "hw/vfio/vfio-common.h"
>  #include "cpu.h"
>  #include "migration/migration.h"
> +#include "migration/vmstate.h"
>  #include "migration/qemu-file.h"
>  #include "migration/register.h"
>  #include "migration/blocker.h"
> @@ -25,6 +28,17 @@
>  #include "trace.h"
>  #include "hw/hw.h"
>  
> +/*
> + * Flags used as delimiter:
> + * 0xffffffff => MSB 32-bit all 1s
> + * 0xef10     => emulated (virtual) function IO
> + * 0x0000     => 16-bits reserved for flags
> + */
> +#define VFIO_MIG_FLAG_END_OF_STATE      (0xffffffffef100001ULL)
> +#define VFIO_MIG_FLAG_DEV_CONFIG_STATE  (0xffffffffef100002ULL)
> +#define VFIO_MIG_FLAG_DEV_SETUP_STATE   (0xffffffffef100003ULL)
> +#define VFIO_MIG_FLAG_DEV_DATA_STATE    (0xffffffffef100004ULL)
> +
>  static inline int vfio_mig_access(VFIODevice *vbasedev, void *val, int count,
>                                    off_t off, bool iswrite)
>  {
> @@ -166,6 +180,65 @@ static int vfio_migration_set_state(VFIODevice *vbasedev, uint32_t mask,
>      return 0;
>  }
>  
> +/* ---------------------------------------------------------------------- */
> +
> +static int vfio_save_setup(QEMUFile *f, void *opaque)
> +{
> +    VFIODevice *vbasedev = opaque;
> +    VFIOMigration *migration = vbasedev->migration;
> +    int ret;
> +
> +    trace_vfio_save_setup(vbasedev->name);
> +
> +    qemu_put_be64(f, VFIO_MIG_FLAG_DEV_SETUP_STATE);
> +
> +    if (migration->region.mmaps) {
> +        qemu_mutex_lock_iothread();
> +        ret = vfio_region_mmap(&migration->region);
> +        qemu_mutex_unlock_iothread();

Please add a comment identifying why the iothread mutex lock is
necessary here.

> +        if (ret) {
> +            error_report("%s: Failed to mmap VFIO migration region %d: %s",
> +                         vbasedev->name, migration->region.nr,

We don't support multiple migration regions, is it useful to include
the region index here?

> +                         strerror(-ret));
> +            error_report("%s: Falling back to slow path", vbasedev->name);
> +        }
> +    }
> +
> +    ret = vfio_migration_set_state(vbasedev, VFIO_DEVICE_STATE_MASK,
> +                                   VFIO_DEVICE_STATE_SAVING);
> +    if (ret) {
> +        error_report("%s: Failed to set state SAVING", vbasedev->name);
> +        return ret;
> +    }

Again, doesn't match the function semantics that success only means the
device is in a non-error state, maybe the one that was asked for.

> +
> +    qemu_put_be64(f, VFIO_MIG_FLAG_END_OF_STATE);

What's the overall purpose of writing these markers into the migration
stream?  vfio_load_state() doesn't do anything with this other than
validate that the end-of-state immediately follows.  Is this a
placeholder for something in the future?

> +
> +    ret = qemu_file_get_error(f);
> +    if (ret) {
> +        return ret;
> +    }
> +
> +    return 0;
> +}
> +
> +static void vfio_save_cleanup(void *opaque)
> +{
> +    VFIODevice *vbasedev = opaque;
> +    VFIOMigration *migration = vbasedev->migration;
> +
> +    if (migration->region.mmaps) {
> +        vfio_region_unmap(&migration->region);
> +    }
> +    trace_vfio_save_cleanup(vbasedev->name);
> +}
> +
> +static SaveVMHandlers savevm_vfio_handlers = {
> +    .save_setup = vfio_save_setup,
> +    .save_cleanup = vfio_save_cleanup,
> +};
> +
> +/* ---------------------------------------------------------------------- */
> +
>  static void vfio_vmstate_change(void *opaque, int running, RunState state)
>  {
>      VFIODevice *vbasedev = opaque;
> @@ -225,6 +298,8 @@ static int vfio_migration_init(VFIODevice *vbasedev,
>                                 struct vfio_region_info *info)
>  {
>      int ret = -EINVAL;
> +    char id[256] = "";
> +    Object *obj;
>  
>      if (!vbasedev->ops->vfio_get_object) {
>          return ret;
> @@ -241,6 +316,22 @@ static int vfio_migration_init(VFIODevice *vbasedev,
>          return ret;
>      }
>  
> +    obj = vbasedev->ops->vfio_get_object(vbasedev);
> +
> +    if (obj) {
> +        DeviceState *dev = DEVICE(obj);
> +        char *oid = vmstate_if_get_id(VMSTATE_IF(dev));
> +
> +        if (oid) {
> +            pstrcpy(id, sizeof(id), oid);
> +            pstrcat(id, sizeof(id), "/");
> +            g_free(oid);
> +        }
> +    }

Here's where vfio_migration_init() starts using vfio_get_object() as I
referenced back on patch 04.  We might as well get the object before
calling vfio_migration_region_init() and then pass the object.  The
conditional branch to handle obj is strange here too, it's fatal if
vfio_migration_region_init() doesn't find an object, why do we handle
it as optional here?  Also, what is this doing?  Comments would be
nice...  Thanks,

Alex

> +    pstrcat(id, sizeof(id), "vfio");
> +
> +    register_savevm_live(id, VMSTATE_INSTANCE_ID_ANY, 1, &savevm_vfio_handlers,
> +                         vbasedev);
>      vbasedev->vm_state = qemu_add_vm_change_state_handler(vfio_vmstate_change,
>                                                            vbasedev);
>      vbasedev->migration_state.notify = vfio_migration_state_notifier;
> diff --git a/hw/vfio/trace-events b/hw/vfio/trace-events
> index bcb3fa7314d7..982d8dccb219 100644
> --- a/hw/vfio/trace-events
> +++ b/hw/vfio/trace-events
> @@ -152,3 +152,5 @@ vfio_migration_probe(const char *name, uint32_t index) " (%s) Region %d"
>  vfio_migration_set_state(const char *name, uint32_t state) " (%s) state %d"
>  vfio_vmstate_change(const char *name, int running, const char *reason, uint32_t dev_state) " (%s) running %d reason %s device state %d"
>  vfio_migration_state_notifier(const char *name, const char *state) " (%s) state %s"
> +vfio_save_setup(const char *name) " (%s)"
> +vfio_save_cleanup(const char *name) " (%s)"