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 <da...@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
