Re: [PATCH v1 4/4] virtio-mem: Support "x-ignore-shared" migration

["Michael S. Tsirkin" <mst@redhat.com>]

On Tue, Jun 20, 2023 at 03:03:54PM +0200, David Hildenbrand wrote:
> To achieve desired "x-ignore-shared" functionality, we should not
> discard all RAM when realizing the device and not mess with
> preallocation/postcopy when loading device state. In essence, we should
> not touch RAM content.
> 
> As "x-ignore-shared" gets set after realizing the device, we cannot
> rely on that. Let's simply skip discarding of RAM on incoming migration.
> Note that virtio_mem_post_load() will call
> virtio_mem_restore_unplugged() -- unless "x-ignore-shared" is set. So
> once migration finished we'll have a consistent state.
> 
> The initial system reset will also not discard any RAM, because
> virtio_mem_unplug_all() will not call virtio_mem_unplug_all() when no
> memory is plugged (which is the case before loading the device state).
> 
> Note that something like VM templating -- see commit b17fbbe55cba
> ("migration: allow private destination ram with x-ignore-shared") -- is
> currently incompatible with virtio-mem and ram_block_discard_range() will
> warn in case a private file mapping is supplied by virtio-mem.
> 
> For VM templating with virtio-mem, it makes more sense to either
> (a) Create the template without the virtio-mem device and hotplug a
>     virtio-mem device to the new VM instances using proper own memory
>     backend.
> (b) Use a virtio-mem device that doesn't provide any memory in the
>     template (requested-size=0) and use private anonymous memory.
> 
> Signed-off-by: David Hildenbrand <david@redhat.com>
> ---
>  hw/virtio/virtio-mem.c | 47 ++++++++++++++++++++++++++++++++++--------
>  1 file changed, 38 insertions(+), 9 deletions(-)
> 
> diff --git a/hw/virtio/virtio-mem.c b/hw/virtio/virtio-mem.c
> index 9f6169af32..b013dfbaf0 100644
> --- a/hw/virtio/virtio-mem.c
> +++ b/hw/virtio/virtio-mem.c
> @@ -18,6 +18,7 @@
>  #include "sysemu/numa.h"
>  #include "sysemu/sysemu.h"
>  #include "sysemu/reset.h"
> +#include "sysemu/runstate.h"
>  #include "hw/virtio/virtio.h"
>  #include "hw/virtio/virtio-bus.h"
>  #include "hw/virtio/virtio-access.h"
> @@ -886,11 +887,23 @@ static void virtio_mem_device_realize(DeviceState *dev, Error **errp)
>          return;
>      }
>  
> -    ret = ram_block_discard_range(rb, 0, qemu_ram_get_used_length(rb));
> -    if (ret) {
> -        error_setg_errno(errp, -ret, "Unexpected error discarding RAM");
> -        ram_block_coordinated_discard_require(false);
> -        return;
> +    /*
> +     * We don't know at this point whether shared RAM is migrated using
> +     * QEMU or migrated using the file content. "x-ignore-shared" will be
> +     * configurated

configurated == configured?

> after realizing the device. So in case we have an
> +     * incoming migration, simply always skip the discard step.
> +     *
> +     * Otherwise, make sure that we start with a clean slate: either the
> +     * memory backend might get reused or the shared file might still have
> +     * memory allocated.
> +     */
> +    if (!runstate_check(RUN_STATE_INMIGRATE)) {
> +        ret = ram_block_discard_range(rb, 0, qemu_ram_get_used_length(rb));
> +        if (ret) {
> +            error_setg_errno(errp, -ret, "Unexpected error discarding RAM");
> +            ram_block_coordinated_discard_require(false);
> +            return;
> +        }
>      }
>  
>      virtio_mem_resize_usable_region(vmem, vmem->requested_size, true);
> @@ -962,10 +975,6 @@ static int virtio_mem_post_load(void *opaque, int version_id)
>      RamDiscardListener *rdl;
>      int ret;
>  
> -    if (vmem->prealloc && !vmem->early_migration) {
> -        warn_report("Proper preallocation with migration requires a newer QEMU machine");
> -    }
> -
>      /*
>       * We started out with all memory discarded and our memory region is mapped
>       * into an address space. Replay, now that we updated the bitmap.
> @@ -978,6 +987,18 @@ static int virtio_mem_post_load(void *opaque, int version_id)
>          }
>      }
>  
> +    /*
> +     * If shared RAM is migrated using the file content and not using QEMU,
> +     * don't mess with preallocation and postcopy.
> +     */
> +    if (migrate_ram_is_ignored(vmem->memdev->mr.ram_block)) {
> +        return 0;
> +    }
> +
> +    if (vmem->prealloc && !vmem->early_migration) {
> +        warn_report("Proper preallocation with migration requires a newer QEMU machine");
> +    }
> +
>      if (migration_in_incoming_postcopy()) {
>          return 0;
>      }
> @@ -1010,6 +1031,14 @@ static int virtio_mem_post_load_early(void *opaque, int version_id)
>          return 0;
>      }
>  
> +    /*
> +     * If shared RAM is migrated using the file content and not using QEMU,
> +     * don't mess with preallocation and postcopy.
> +     */
> +    if (migrate_ram_is_ignored(rb)) {
> +        return 0;
> +    }
> +
>      /*
>       * We restored the bitmap and verified that the basic properties
>       * match on source and destination, so we can go ahead and preallocate
> -- 
> 2.40.1