Re: [Xen-devel] [PATCH] x86/amd: don't set X86_BUG_SYSRET_SS_ATTRS if forced to zero

[Juergen Gross <jgross@xxxxxxxx>]

On 27/04/17 00:04, Borislav Petkov wrote:
> On Wed, Apr 26, 2017 at 08:24:12PM +0200, Juergen Gross wrote:
>> I'm not feeling strong about it. So if you want to test for
>> X86_FEATURE_XENPV to avoid setting X86_BUG_SYSRET_SS_ATTRS I'm fine
>> with it.
>>
>> Will send V2 with that change.
> 
> And remove the corresponding
> 
>       clear_cpu_bug(c, X86_BUG_SYSRET_SS_ATTRS);
> 
> in xen_set_cpu_features().

Okay, you are right, we can omit this one now.

> So that we can set it once, only on !XENPV feature set.
> 
> /me looks again at the code...
> 
> Gah, except that we do
> 
>       set_cpu_cap(c, X86_FEATURE_XENPV);
> 
> and that runs as part of init_hypervisor() and it runs *after* c_init().

No, this is called by xen_start_kernel(), long before c_init().

> So, back to square one. :-\
> 
> So lemme try to explain again what I mean:
> 
> I'd like to have a generic way of detecting whether I'm running as a xen
> guest at ->c_init() time and depending on the result of that detection,
> to set X86_BUG_SYSRET_SS_ATTRS or not set it.
> 
> Does that make more sense?

This does make sense and it is working, as Sander could confirm.


Juergen

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