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

["Richard W.M. Jones" <rjones@redhat.com>]

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

[...]
> > > > +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 have a v3 which considerably simplifies the specification &
implementation which I'll post very soon.

Rich.

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