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

[Li, Xin3]

> 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.
> 

Got to say, nothing is more convincing than strong data.
Xin

> 
> Juergen