[PATCH] vhost-user-blk-server: fix size_max=0 breaking Windows guests

[Max Makarov <maxpain177@gmail.com>]

The QSD vhost-user-blk export sets size_max=0 in the virtio config,
but the vhost-user-blk frontend unconditionally advertises
VIRTIO_BLK_F_SIZE_MAX. This creates a spec-violating state: the
feature bit tells the guest that size_max is valid, but the value
is 0.

Native virtio-blk-pci does not advertise VIRTIO_BLK_F_SIZE_MAX when
size_max=0, so guests never see this contradiction there.

The Windows viostor driver trusts the feature bit and uses size_max=0
in its MaximumTransferLength calculation, producing malformed
scatter-gather requests. The disk appears completely empty to Windows
(no GPT, no partitions in diskpart), causing INACCESSIBLE_BOOT_DEVICE
BSOD. Linux is unaffected because its virtio_blk driver falls back to
PAGE_SIZE when size_max=0.

Set size_max to 4096 (PAGE_SIZE) to provide a valid segment size limit
that matches what Linux assumes as a fallback. This fixes Windows
Server 2025 (and likely all Windows versions) booting from
vhost-user-blk.

Buglink: https://github.com/spdk/spdk/issues/3199
Buglink: https://github.com/virtio-win/kvm-guest-drivers-windows/issues/692
Signed-off-by: Max Makarov <maxpain@linux.com>
---
 block/export/vhost-user-blk-server.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/block/export/vhost-user-blk-server.c b/block/export/vhost-user-blk-server.c
index e89422b..827502d 100644
--- a/block/export/vhost-user-blk-server.c
+++ b/block/export/vhost-user-blk-server.c
@@ -240,7 +240,7 @@ vu_blk_initialize_config(BlockDriverState *bs,
     config->capacity =
         cpu_to_le64(bdrv_getlength(bs) >> VIRTIO_BLK_SECTOR_BITS);
     config->blk_size = cpu_to_le32(blk_size);
-    config->size_max = cpu_to_le32(0);
+    config->size_max = cpu_to_le32(4096);
     config->seg_max = cpu_to_le32(128 - 2);
     config->min_io_size = cpu_to_le16(1);
     config->opt_io_size = cpu_to_le32(1);
-- 
2.53.0