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

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

On Tue, Sep 08, 2026 at 06:06:14AM -0400, Michael S. Tsirkin wrote:
> 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?

Indeed you're right ... and this is very confusing.

qemu lets you set both serial and device_id, eg:

$ qemu-system-x86_64 -machine accel=kvm:tcg -cpu max -m 4096 \
  -device virtio-scsi-pci,id=scsi0 \
  -drive id=hd1,file=$HOME/tmp/vms/rhel-10.qcow2,format=qcow2,if=none \
  -device scsi-hd,bus=scsi0.0,drive=hd1,device_id=ABCDEFGHIJ0123456789abcdefghij0123456789ABCDEFGHIJ0123456789abcdefghij0123456789ABCDEFGHIJ0123456789abcdefghij0123456789ABCDEFGHIJ0123456789abcdefghij0123456789ABCDEFGHIJ0123456789abcdefghij0123456789ABCDEFGHIJ0123456789abcdefghij0123456789,serial=123456789012345678901234567890 

'serial' is exposed through VPD page 0x80 and is limited to 36 chars.

'device_id' is exposed through VPD page 0x83 and is limited to 256-8
bytes.

udev seems to use page 0x80 (not 0x83) to generate symlinks.

Anyway 20 chars is < 36 chars so the need for longer serials for
virtio-blk remains.

Rich.

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