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Jones" To: virtio-comment@lists.linux.dev Cc: "Michael S . Tsirkin" Subject: [PATCH v3 1/1] device-types/blk/description.tex: Allow longer device IDs to be returned Date: Tue, 8 Sep 2026 10:40:50 +0100 Message-ID: <20260908094050.915173-2-rjones@redhat.com> In-Reply-To: <20260908094050.915173-1-rjones@redhat.com> References: <20260908094050.915173-1-rjones@redhat.com> Precedence: bulk X-Mailing-List: virtio-comment@lists.linux.dev List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 X-Scanned-By: MIMEDefang 3.0 on 10.30.177.12 X-Mimecast-Spam-Score: 0 X-Mimecast-MFC-PROC-ID: nG8jPGOI9eojlWDT0oOdzddrH5mwFdgsCHk-YYQmIc0_1788860454 X-Mimecast-Originator: redhat.com Content-Transfer-Encoding: 8bit content-type: text/plain; charset="US-ASCII"; x-default=true SCSI-based paravirtualized block devices including virtio-scsi and VMware's pvscsi allow longer device IDs. This presents an issue when we change the backing of a disk from one type to another, eg from pvscsi to virtio-blk, or virtio-scsi to virtio-blk. The longer device ID has to be truncated to 20 bytes. This results in guest visible changes, notably /dev/disk/by-id/ paths are different, so any mountpoints or configuration files that use these paths will break. Therefore extend virtio-blk to allow longer device IDs. I chose 247 bytes (ASCII chars) as the new limit since SCSI serials can in theory be this long. Real serials will be much shorter than this; the aim is to allow UUIDs to be preserved which would use 32 or 36 ASCII chars. A new feature bit is introduced to indicate that long device IDs are supported. Signed-off-by: Richard W.M. Jones --- device-types/blk/description.tex | 27 ++++++++++++++++++++++++--- 1 file changed, 24 insertions(+), 3 deletions(-) diff --git a/device-types/blk/description.tex b/device-types/blk/description.tex index 3b3a4e7..aa3ec66 100644 --- a/device-types/blk/description.tex +++ b/device-types/blk/description.tex @@ -73,6 +73,9 @@ \subsection{Feature bits}\label{sec:Device Types / Block Device / Feature bits} VIRTIO_BLK_REQ_FLAG_OUT_FUA flag in the \field{flags} bitfield of the \field{virtio_blk_req} structure for VIRTIO_BLK_T_OUT requests. +\item[VIRTIO_BLK_F_LONG_ID (20)] Device supports long device ID + strings. + \end{description} \subsubsection{Legacy Interface: Feature bits}\label{sec:Device Types / Block Device / Feature bits / Legacy Interface: Feature bits} @@ -516,8 +519,23 @@ \subsection{Device Operation}\label{sec:Device Types / Block Device / Device Ope zeroes. VIRTIO_BLK_T_GET_ID requests fetch the device ID string from the device into -\field{data}. The device ID string is a NUL-padded ASCII string up to 20 bytes -long. If the string is 20 bytes long then there is no NUL terminator. +\field{data}. The device ID string is an ASCII string which can be +up to 247 bytes long. + +If VIRTIO_BLK_F_LONG_ID was not negotiated, VIRTIO_BLK_T_GET_ID +fetches the first 20 bytes of the device ID string. If the ID is +shorter than 20 bytes, then the response is padded with NUL bytes so +its length is 20 bytes. (Note that if the ID is 20 bytes or longer, +this means the response will not be NUL terminated.) + +If VIRTIO_BLK_F_LONG_ID was negotiated, VIRTIO_BLK_T_GET_ID fetches +the complete device ID string. The response is always 248 bytes long, +padded to this length with NUL bytes. Since the longest permitted +device ID string is 247 bytes, the response is NUL terminated. + +Although 247 byte device ID strings are allowed, there may be +interoperability problems if strings longer than 128 bytes are used. +It is also advisable to use only 7 bit ASCII characters. The \field{data} used for VIRTIO_BLK_T_GET_LIFETIME requests is populated by the device, and is of the form @@ -911,7 +929,10 @@ \subsection{Device Operation}\label{sec:Device Types / Block Device / Device Ope virtio_blk_discard_write_zeroes for VIRTIO_BLK_T_DISCARD, VIRTIO_BLK_T_SECURE_ERASE and VIRTIO_BLK_T_WRITE_ZEROES requests. -The length of \field{data} MUST be 20 bytes for VIRTIO_BLK_T_GET_ID requests. +For VIRTIO_BLK_T_GET_ID requests the length of \field{data} depends +on the feature VIRTIO_BLK_F_LONG_ID. If VIRTIO_BLK_F_LONG_ID was +negotiated the length of \field{data} MUST be 248 bytes, else +it MUST be 20 bytes. VIRTIO_BLK_T_DISCARD requests MUST NOT contain more than \field{max_discard_seg} struct virtio_blk_discard_write_zeroes segments in -- 2.55.0