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

[H. Peter Anvin]

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.