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Tsirkin" , Kevin Wolf , Hanna Reitz , Max Makarov Subject: [PATCH] vhost-user-blk-server: fix size_max=0 breaking Windows guests Date: Mon, 30 Mar 2026 18:13:13 +0300 Message-ID: <20260330151313.66318-1-maxpain@linux.com> X-Mailer: git-send-email 2.50.1 MIME-Version: 1.0 Content-Transfer-Encoding: quoted-printable Received-SPF: pass (zohomail.com: domain of gnu.org designates 209.51.188.17 as permitted sender) client-ip=209.51.188.17; envelope-from=qemu-devel-bounces+importer=patchew.org@nongnu.org; helo=lists.gnu.org; Received-SPF: pass client-ip=2a00:1450:4864:20::129; envelope-from=maxpain177@gmail.com; helo=mail-lf1-x129.google.com X-Spam_score_int: -17 X-Spam_score: -1.8 X-Spam_bar: - X-Spam_report: (-1.8 / 5.0 requ) BAYES_00=-1.9, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, FREEMAIL_ENVFROM_END_DIGIT=0.25, FREEMAIL_FROM=0.001, RCVD_IN_DNSWL_NONE=-0.0001, SPF_HELO_NONE=0.001, SPF_PASS=-0.001 autolearn=unavailable autolearn_force=no X-Spam_action: no action X-Mailman-Approved-At: Mon, 30 Mar 2026 12:42:36 -0400 X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: qemu development List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: qemu-devel-bounces+importer=patchew.org@nongnu.org Sender: qemu-devel-bounces+importer=patchew.org@nongnu.org X-ZohoMail-DKIM: pass (identity @gmail.com) X-ZM-MESSAGEID: 1774888989490158500 Content-Type: text/plain; charset="utf-8" The QSD vhost-user-blk export sets size_max=3D0 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=3D0, so guests never see this contradiction there. The Windows viostor driver trusts the feature bit and uses size_max=3D0 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=3D0. 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 --- 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 =3D cpu_to_le64(bdrv_getlength(bs) >> VIRTIO_BLK_SECTOR_BITS); config->blk_size =3D cpu_to_le32(blk_size); - config->size_max =3D cpu_to_le32(0); + config->size_max =3D cpu_to_le32(4096); config->seg_max =3D cpu_to_le32(128 - 2); config->min_io_size =3D cpu_to_le16(1); config->opt_io_size =3D cpu_to_le32(1); --=20 2.53.0