Re: [PATCH V2] xen/arm: dm: Drop XEN_DMOP_get_ioreq_server_info from supported

[Julien Grall <julien@xxxxxxx>]

On 02/09/2025 13:10, Orzel, Michal wrote:
> On 02/09/2025 13:54, Julien Grall wrote:
> > Hi,
> >
> > On 02/09/2025 11:18, Orzel, Michal wrote:
> > > On 02/09/2025 11:49, Oleksandr Tyshchenko wrote:
> > > > The said sub-op is not supported on Arm, since it:
> > > >    - does not support the buffered emulation (so bufioreq_port/bufioreq_gfn
> > > >      cannot be returned), please refer to ioreq_server_create()
> > > >    - does not support "legacy" mechanism of mapping IOREQ Server
> > > >      magic pages (so ioreq_gfn/bufioreq_gfn cannot be returned), please
> > > >      refer to arch_ioreq_server_map_pages(). On Arm, only the Acquire
> > > >      Resource infrastructure is used to query and map the IOREQ Server pages.
> > > >
> > > > Signed-off-by: Oleksandr Tyshchenko <oleksandr_tyshchenko@xxxxxxxx>
> > > Reviewed-by: Michal Orzel <michal.orzel@xxxxxxx>
> > >
> > > Could we perhaps add a Fixes tag here pointing to the commit introducing these
> > > DM ops and thus add this patch for this release? Not sure what others think.
> > Fixes usually implies a bug and I don't see what bug we are solving. In
> > fact, I don't understand why we are trying to remove the subop...
> Hmm, the issue is that the subop that is not supported at the moment is listed
> as supported in the public header.
[...]

> As for the code, from safety perspective if this subop is listed explicilty in 
> Arm's
> dm.c, we would need to write a separate test case and test to cover it that at
> the end, still returns -EOPNOTSUPP.
Why do you think it is not supported? AFAICT, it is still possible to 
pass XEN_DMOP_nognfs to figure out whwether bufioreq is currently 
available. The error code would be 0 not -EOPNOTSUPP.
> I think if we mistakenly mention sth as> supported in e.g. SUPPORT.md 
we would have no issues adding a Fixes tag. There
> are many cases where Fixes was used just to change something in a 
comment, so
> I'm having a hard time reasoning about when it's appropriate to use it.
I think what we would want is "Amends". This is currently proposed by [1].

[1] 
https://lore.kernel.org/xen-devel/e7c99116-f6a9-43e1-80ae-b3a4d44857b7@xxxxxxx/T/#t
> ~Michal
--
Julien Grall