The QSD vhost-user-blk backend advertises VIRTIO_BLK_F_SIZE_MAX but
sets size_max=0 in config, creating a contradictory state. The feature
bit tells the guest that size_max is valid, but the value is zero.
Windows viostor trusts the feature bit, reads size_max=0, and uses it
in scatter-gather calculations producing zero-length segments that hang
I/O. The disk appears empty (no GPT, no partitions), causing
INACCESSIBLE_BOOT_DEVICE BSOD. Linux tolerates this because
blk_validate_limits() silently corrects max_segment_size=0 to
BLK_MAX_SEGMENT_SIZE (65536).
This affects any VMM using QSD as a vhost-user-blk backend (QEMU,
Cloud Hypervisor, etc.), since both pass through SIZE_MAX from the
backend to the guest via feature intersection.
Remove VIRTIO_BLK_F_SIZE_MAX from the backend feature bits. The
size_max config field remains zero but is now ignored by guests since
the feature is not negotiated.
Signed-off-by: Max Makarov <maxpain@linux.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Message-Id: <20260331152856.1802-1-maxpain@linux.com>
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