Re: [PATCH 1/1] device-types/blk/description.tex: Allow longer device IDs to be returned

["Michael S. Tsirkin" <mst@redhat.com>]

On Tue, Sep 08, 2026 at 10:35:34AM +0100, Richard W.M. Jones wrote:
> On Mon, Sep 07, 2026 at 03:27:25PM -0400, Michael S. Tsirkin wrote:
> > On Mon, Sep 07, 2026 at 03:02:24PM +0100, Richard W.M. Jones wrote:
> > > On Mon, Sep 07, 2026 at 03:56:56AM -0400, Michael S. Tsirkin wrote:
> > > > why 247? It feels like a lot, especially given we are padding. I'd say
> > > > 128 maybe.  This way it fits in a single pci express packet on most
> > > > systems.
> > > 
> > > virtio-scsi supports, at least in theory, 248 bytes, see:
> > > 
> > > https://gitlab.com/qemu/qemu/-/blob/cacd3462963a0a4f5bab4263ce79c2aa4b32692d/hw/scsi/scsi-disk.c#L708
> > > 
> > > This is chosen because of the maximum page size of a SCSI VPD (256)
> > > minus the 8 bytes used for the header in VPD page 0x83.
> > 
> > Can I see a spec which says that is the maximum size?
> 
> I don't think virtio-scsi is following a spec,


But you referred to SCSI VPD and said it has a limit of 256
and an 8 byte header. I can't figure out where this is from?

> but it's clear from the
> source code that the limit is 256-8, see the link above.
> 
> [...]


Hmm. I see MAX_SERIAL_LEN which is 36? And  MAX_SERIAL_LEN_FOR_DEVID
which is 20?


> > > > > +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.)
> > > > 
> > > > Why first specifically?
> > > 
> > > For backwards compatibility (assuming I understand the question).
> > > qemu's current behaviour is to truncate.
> > 
> > That's just a qemu command line quirk then.
> > At least from spec POV it isn't - the id must be <= 20.
> 
> Isn't the virtio spec also there (partly) to clarify and formalise
> what existing implementations actually do?  qemu truncates.
> Firecracker truncates:
> 
> https://github.com/firecracker-microvm/firecracker/blob/7699746649826d1dfcdde626b3131bac08f28e0d/src/vmm/src/devices/virtio/block/virtio/device.rs#L159
> 
> libkrun seems to be using the same code as Firecracker, so it also
> truncates:
> 
> https://github.com/libkrun/libkrun/blob/413a011760b7ee61493fb0073f7810319ca6a181/src/devices/src/virtio/block/device.rs#L170


I can easily imagine a situation where someone prefixes the serial
with "virtio-" or whatever extra data, and shows the suffix
as the short id. Or hashes the short id. Or whatever.


> I have a v3 which considerably simplifies the specification &
> implementation which I'll post very soon.
> 
> Rich.
> 
> -- 
> Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
> Read my programming and virtualization blog: http://rwmj.wordpress.com
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