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

[Juergen Gross <jgross@suse.com>]

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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.

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.


Juergen


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