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

[Juergen Gross]

On 19.10.22 20:01, H. Peter Anvin wrote:
> On October 19, 2022 10:45:07 AM PDT, "Li, Xin3" <xin3.li@xxxxxxxxx> 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.
> >
> > Xin
> >
> > > Juergen
>
> Most things don't frob the paravirt list.
>
> Maybe we should make the paravirt frobbing a separate patch, at it is separable.

Works for me.


Juergen

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