Re: [PATCH v4 5/5] x86/gsseg: use the LKGS instruction if available for load_gs_index()

[Juergen Gross]

On 20.10.22 07:58, Li, Xin3 wrote:
> > On 19.10.22 19:45, Li, Xin3 wrote:
> > > > > +static inline void __init lkgs_init(void) { #ifdef
> > > > > +CONFIG_PARAVIRT_XXL #ifdef CONFIG_X86_64
> > > > > +	if (cpu_feature_enabled(X86_FEATURE_LKGS))
> > > > > +		pv_ops.cpu.load_gs_index = native_lkgs;
> > > >
> > > > For this to work correctly when running as a Xen PV guest, you need
> > > > to add
> > > >
> > > > 	setup_clear_cpu_cap(X86_FEATURE_LKGS);
> > > >
> > > > to xen_init_capabilities() in arch/x86/xen/enlighten_pv.c, as
> > > > otherwise the Xen specific .load_gs_index vector will be overwritten.
> > >
> > > Yeah, we definitely should add it to disable LKGS in a Xen PV guest.
> > >
> > > So does it mean that the Xen PV uses a black list during feature detection?
> > > If yes then new features are often required to be masked with an
> > > explicit call to setup_clear_cpu_cap.
> > >
> > > Wouldn't a white list be better?
> > > Then the job is more just on the Xen PV side, and it can selectively
> > > enable a new feature, sometimes with Xen PV specific handling code added.
> >
> > This is not how it works. Feature detection is generic code, so we'd need to
> > tweak that for switching to a whitelist.
>
> Yes, a Xen PV guest is basically a Linux system.  However IIRC, the Xen PV
> CPUID is para-virtualized, so it's Xen hypervisor's responsibility to decide
> features exposed to a Xen PV guest.  No?

In theory you are right, of course.

OTOH the Xen PV interface has a long and complicated history, and we have to
deal with old hypervisor versions, too.

> > Additionally most features don't require any Xen PV specific handling. This is
> > needed for some paravirtualized privileged operations only. So switching to a
> > whitelist would add more effort.
>
> LKGS is allowed only in ring 0, thus only Xen hypervisor could use it.

Right, it would be one of the features where a whitelist would be nice.

OTOH today only 11 features need special handling in Xen PV guests, while
the rest of more than 300 features doesn't.


Juergen

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