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

[Li, Xin3]

> > +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