Re: [PATCH v3 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 04:46:40PM +0100, Richard W.M. Jones wrote:
> On Tue, Sep 08, 2026 at 11:14:46AM -0400, Stefan Hajnoczi wrote:
> > On Tue, Sep 08, 2026 at 10:40:50AM +0100, Richard W.M. Jones wrote:
> > > +Although 247 byte device ID strings are allowed, there may be
> > > +interoperability problems if strings longer than 128 bytes are used.
> > 
> > Can you be more specific? Is the concern that the guest software stack
> > above of the driver may not be prepared for serial strings longer than
> > 128 bytes?
> 
> I'll add more detail in the next version, but in brief the problems
> are twofold:
> 
> (1) Windows supports up to 128 "characters" (they're not precise but
> they probably mean Unicode codepoints from the Basic Multilingual Plane):
> 
> https://learn.microsoft.com/en-us/windows-hardware/drivers/ddi/storport/ns-storport-stor_serial_number
> 
> (2) When udev tries to encode a very long serial into a
> /dev/disk/by-id path you end up uncomfortably close to NAME_MAX (255).
> 
> > > +It is also advisable to use only 7 bit ASCII characters.
> > 
> > Versus "The device ID string is an ASCII string which can be up to 247
> > bytes long" earlier in this patch. Is ASCII a "SHOULD" or a "MUST"?
> 
> It's "SHOULD".  I can't see any good coming from trying to use
> anything except letters, numbers and dashes in a serial, but both
> Linux and Windows (see above) could in theory handle Unicode.
> 
> Rich.


Linux actually has code to try and handle that. unicode
is actually the less problematic part - ascii is
full of pitfalls, eith things like
slashes are much weirder - e.g. udev will create
a subdirectory with slashes. what will windows do?
and what will it do with backslashes? /me shrugs

> -- 
> Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
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