Re: [PATCH] xen: Remove config dependency in XEN_PRIVCMD definition

[Stefano Stabellini <sstabellini@xxxxxxxxxx>]

On Wed, 9 Oct 2024, Jan Beulich wrote:
> On 09.10.2024 08:20, Jiqian Chen wrote:
> > Commit 2fae6bb7be32 ("xen/privcmd: Add new syscall to get gsi from dev")
> > adds a weak reverse dependency to the config XEN_PRIVCMD definition, its
> > purpose is to pass the combination of compilation that CONFIG_XEN_PRIVCMD=y
> > and CONFIG_XEN_PCIDEV_BACKEND=m, because in that combination, xen-pciback
> > is compiled as a module but xen-privcmd is built-in, so xen-privcmd can't
> > find the implementation of pcistub_get_gsi_from_sbdf.
> > 
> > But that dependency causes xen-privcmd can't be loaded on domU, because
> > dependent xen-pciback is always not be loaded successfully on domU.
> > 
> > To solve above problem and cover original commit's requirement, just remove
> > that dependency, because the code "IS_REACHABLE(CONFIG_XEN_PCIDEV_BACKEND)"
> > of original commit is enough to meet the requirement.
> > 
> > Fixes: 2fae6bb7be32 ("xen/privcmd: Add new syscall to get gsi from dev")
> > Signed-off-by: Jiqian Chen <Jiqian.Chen@xxxxxxx>
> 
> This lacks a Reported-by:.
> 
> > --- a/drivers/xen/Kconfig
> > +++ b/drivers/xen/Kconfig
> > @@ -261,7 +261,6 @@ config XEN_SCSI_BACKEND
> >  config XEN_PRIVCMD
> >     tristate "Xen hypercall passthrough driver"
> >     depends on XEN
> > -   imply XEN_PCIDEV_BACKEND
> >     default m
> >     help
> >       The hypercall passthrough driver allows privileged user programs to
> 
> The report wasn't about a build problem, but a runtime one. Removing the
> dependency here doesn't change anything in the dependency of xen-privcmd
> on xen-pciback, as the use of pcistub_get_gsi_from_sbdf() continues to
> exist.
>
> Consider the case of XEN_PCIDEV_BACKEND=m and XEN_PRIVCMD=m, which
> I guess is what Marek is using in his config. Both drivers are available
> in such a configuration, yet loading of xen-privcmd then requires to
> load xen-pciback first. And that latter load attempt will fail in a DomU.
> The two drivers simply may not have any dependency in either direction.

The idea is that there should be no hard dependency on
pcistub_get_gsi_from_sbdf(). If it is available, the service will be
used, otherwise an error will be reported.

The problem is that IS_REACHABLE is a compile-time check. What we need
is a runtime check instead. Maybe symbol_get or try_module_get ?