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Jones" To: "Michael S. Tsirkin" Cc: virtio-comment@lists.linux.dev Subject: Re: [PATCH v3 1/1] device-types/blk/description.tex: Allow longer device IDs to be returned Message-ID: <20260908115108.GV1436@redhat.com> References: <20260908094050.915173-1-rjones@redhat.com> <20260908094050.915173-2-rjones@redhat.com> <20260908060804-mutt-send-email-mst@kernel.org> Precedence: bulk X-Mailing-List: virtio-comment@lists.linux.dev List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 In-Reply-To: <20260908060804-mutt-send-email-mst@kernel.org> User-Agent: Mutt/1.5.21 (2010-09-15) X-Scanned-By: MIMEDefang 3.0 on 10.30.177.12 X-Mimecast-Spam-Score: 0 X-Mimecast-MFC-PROC-ID: GlJWbGnwPaI4UVo4hJRqc5dKMP7YNJR0kMBzWCdqU8M_1788868270 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=us-ascii Content-Disposition: inline On Tue, Sep 08, 2026 at 06:57:08AM -0400, Michael S. Tsirkin wrote: > Thanks for the patch! Yes something to improve: > > On Tue, Sep 08, 2026 at 10:40:50AM +0100, Richard W.M. Jones wrote: > > 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. > > Judging by the below, it does not have to be ASCII? just a nul > terminated string? > > > 247 is just a very weird limit. > So we get another > arbitrary number drivers need to worry about. Ugh. > I just don't get all the references > to scsi vpd or whatever. It's your motivation I get that > but we should not set policy. > > Let's just simply limit the string to 255 bytes and > make data 255 bytes too? I wonder actually if 128 is better. It fits within the Windows limit and it doesn't hurt for Linux either because of NAME_MAX limitations that you would encounter with udev. > Then with the status byte the write buffer fits in a 256 pcie packet. > > Maybe others feel differently. > > > + > > +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.) > > But the driver can not both negotiate and not negotiate > VIRTIO_BLK_F_LONG_ID. Interesting .. so I think I misunderstood how feature negotiation works. I _thought_ that the driver sends back features it understands, via the features array: https://github.com/torvalds/linux/blob/28924df2a08f440c73991b83028032c901de2ae4/drivers/block/virtio_blk.c#L1669 and therefore the host side can see if the driver supports VIRTIO_BLK_F_LONG_ID. Is that not how it works? > So what does this requirement that the two configs share > a prefix buy us, practically? > > Why don't we just say that the ID is up to either 20 bytes or > 255 bytes depending on the feature negotiation? We can just drop the "first" language. > > + > > +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. > > Let's just handle short and long forms consistently please. > If the string fills all of data then it is not terminated. end of story. > > All the subtle differences are just a source for confusion. Can we not > define something like "maximum ID length" and then use it everywhere, > instead of all the duplication? Sure. > > + > > +Although 247 byte device ID strings are allowed, there may be > > +interoperability problems if strings longer than 128 bytes are used. > > I think you mean 127? so it's 128 with the NUL. > > > +It is also advisable to use only 7 bit ASCII characters. > > ASCII == 7 bit. Maybe you mean printable characters? > > > Pls add both these recommendations in the conformance section. OK. Rich. -- Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones Read my programming and virtualization blog: http://rwmj.wordpress.com nbdkit - Flexible, fast NBD server with plugins https://gitlab.com/nbdkit/nbdkit