[PATCH v2] vhost-user-blk: do not force VIRTIO_BLK_F_SIZE_MAX

[PATCH v2] vhost-user-blk: do not force VIRTIO_BLK_F_SIZE_MAX

[Max Makarov]

vhost_user_blk_get_features() unconditionally advertises
VIRTIO_BLK_F_SIZE_MAX regardless of what the backend reports in
its config. The built-in QSD vhost-user-blk backend sets
size_max=0, creating a contradictory state where the feature bit
tells the guest that size_max is valid but the value is zero.

The in-process virtio-blk device does not advertise
VIRTIO_BLK_F_SIZE_MAX (it also sets size_max=0 in config), so
guests never see this contradiction with native virtio-blk.

Linux tolerates size_max=0 because blk_validate_limits() silently
corrects max_segment_size=0 to BLK_MAX_SEGMENT_SIZE (65536).
Windows viostor, however, trusts the feature bit and uses the raw
size_max=0 in its scatter-gather calculations, producing
zero-length segments that hang I/O. The disk appears empty to
Windows (no GPT, no partitions), causing INACCESSIBLE_BOOT_DEVICE.

Stop force-adding VIRTIO_BLK_F_SIZE_MAX in the frontend. The
feature remains in user_feature_bits[], so backends that properly
advertise it with a valid config value will still work.

Signed-off-by: Max Makarov <maxpain@linux.com>
---
v2:
 - Remove the VIRTIO_BLK_F_SIZE_MAX feature bit from the frontend
   instead of setting size_max=4096 in the backend config [Kevin Wolf]
 - Fix incorrect claim about Linux PAGE_SIZE fallback
 - Drop unrelated Buglink tags
---
 hw/block/vhost-user-blk.c | 1 -
 1 file changed, 1 deletion(-)

diff --git a/hw/block/vhost-user-blk.c b/hw/block/vhost-user-blk.c
index c151e83..7ba6562 100644
--- a/hw/block/vhost-user-blk.c
+++ b/hw/block/vhost-user-blk.c
@@ -273,7 +273,6 @@ static uint64_t vhost_user_blk_get_features(VirtIODevice *vdev,
     VHostUserBlk *s = VHOST_USER_BLK(vdev);
 
     /* Turn on pre-defined features */
-    virtio_add_feature(&features, VIRTIO_BLK_F_SIZE_MAX);
     virtio_add_feature(&features, VIRTIO_BLK_F_SEG_MAX);
     virtio_add_feature(&features, VIRTIO_BLK_F_GEOMETRY);
     virtio_add_feature(&features, VIRTIO_BLK_F_TOPOLOGY);
-- 
2.53.0