On 2026/07/28 16:04, Marc-André Lureau wrote:
Hi
On Tue, Jul 28, 2026 at 10:26 AM Akihiko Odaki
<[email protected]> wrote:
On 2026/07/27 18:30, Daniel P. Berrangé wrote:
On Mon, Jul 27, 2026 at 06:23:25PM +0900, Akihiko Odaki wrote:
The blob size is encoded as a 64-bit integer in the virtio protocol, but
it is encoded as a 32-bit integer in the migration stream, and
a large blob whose size cannot be expressed in a 32-bit integer will
be corrupted after migration. Deny creating such large blobs.
Is there a valid use case for such large blobs ? If so, rather than
rejecting the blob, we could dynamically add a migration blocker.
That would allow the valid use cases, while preventing the migration
problem.
For the QEMU code path changed here, there should be no practical use
case for a blob larger than 4 GiB. The non-virgl implementation uses
blobs as scanout or cursor backing. Such a size is excessive for those uses.
A dynamic migration blocker would also let the guest make an otherwise
migratable VM non-migratable at runtime merely by creating such a
resource. This could interfere with host-level VM management. So I think
rejecting sizes that the current migration format cannot represent is
better for now.
What's the difference? The dynamic blocker would simply happen earlier
or be queryable, no?
The difference I had in mind was that the guest could make migration
unavailable after the VM had started in an unexpected way for the user
who manages the host.
But I now remember that, if QEMU is started with -only-migratable,
migrate_add_blocker() fails, so resource creation can be rejected in
that configuration. It also fails while a migration or snapshot is
already in progress.
Now I feel safer with the dynamic blocker. I will change the patch to
keep a migration blocker installed while any such blob exists.
Regards,
Akihiko Odaki
If a concrete use case emerges, extending the migration format would be
preferable to making migratability depend on guest-controlled state.
Regards,
Akihiko Odaki
Fixes: f66767f75c9c ("virtio-gpu: add virtio-gpu/blob vmstate subsection")
Fixes: 10b9ddbc83b9 ("Revert "virtio-gpu: block migration of VMs with
blob=true"")
Signed-off-by: Akihiko Odaki <[email protected]>
---
Changes in v2:
- Add the Fixes: tag for f66767f75c9c.
- Add a TODO to encode blob_size as 64 bits in virtio-gpu/blob
VMState version 2.
- Link to v1:
https://lore.kernel.org/qemu-devel/[email protected]
---
hw/display/virtio-gpu.c | 15 +++++++++++++++
1 file changed, 15 insertions(+)
diff --git a/hw/display/virtio-gpu.c b/hw/display/virtio-gpu.c
index 718ba3039290..987b1b3ea860 100644
--- a/hw/display/virtio-gpu.c
+++ b/hw/display/virtio-gpu.c
@@ -352,6 +352,14 @@ static void virtio_gpu_resource_create_blob(VirtIOGPU *g,
return;
}
+ if (cblob.size > UINT32_MAX) {
+ qemu_log_mask(LOG_GUEST_ERROR,
+ "%s: blob too large (%" PRIu64 "> %" PRIu32 ")\n",
+ __func__, cblob.size, UINT32_MAX);
+ cmd->error = VIRTIO_GPU_RESP_ERR_OUT_OF_MEMORY;
+ return;
+ }
+
if (virtio_gpu_find_resource(g, cblob.resource_id)) {
qemu_log_mask(LOG_GUEST_ERROR, "%s: resource already exists %d\n",
__func__, cblob.resource_id);
@@ -1435,7 +1443,14 @@ static int virtio_gpu_blob_save(QEMUFile *f, void
*opaque, size_t size,
}
assert(!res->image);
qemu_put_be32(f, res->resource_id);
+
+ /*
+ * The migration stream encodes blob_size as a 32-bit integer for
+ * compatibility though virtio encodes it as a 64-bit integer. A newer
+ * version of the migration stream should encode it as a 64-bit
integer.
+ */
qemu_put_be32(f, res->blob_size);
+
qemu_put_be32(f, res->iov_cnt);
for (i = 0; i < res->iov_cnt; i++) {
qemu_put_be64(f, res->addrs[i]);
---
base-commit: 006a22cb26998998385b104db1ff9466ef2f3153
change-id: 20260725-blob-8a6a1a85601a
Best regards,
--
Akihiko Odaki <[email protected]>
With regards,
Daniel
