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

[Jürgen Groß <jgross@xxxxxxxx>]

On 10.10.24 09:07, Chen, Jiqian wrote:
> On 2024/10/10 13:39, Jürgen Groß wrote:
> > On 10.10.24 00:46, Stefano Stabellini wrote:
> > > 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 ?
> > This is a rather clumsy solution IMO.
> >
> > I'm seeing the following solutions:
> >
> > 1. Don't fail to to load the pciback driver in a DomU, but only disable
> >     any functionality.
> >
> > 2. Move the drivers/xen/xen-pciback/pci_stub.c functionality in a module
> >     of its own, allowing the privcmd driver to be loaded without needing
> >     the rest of pciback.
> >
> > 3. Add a hook to e.g. drivers/xen/pci.c instead for calling of
> >     pcistub_get_gsi_from_sbdf() directly. pciback could register the real
> >     call address. If pciback isn't loaded, the hook can return an error.
> >     This would remove the explicit dependency of privcmd on pciback.
> >
> > I'd prefer the 3rd variant.
> Thanks, I will send changes in v2 soon.
> And I prefer to add the hook in drivers/xen/acpi.c
Agreed.


Juergen

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