Re: [Xen-devel] [PATCH 2/3] x86/xsaves: fix overwriting between non-lazy/lazy xsave[sc]

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 24.02.16 at 08:05, <shuai.ruan@xxxxxxxxxxxxxxx> wrote:
> On Mon, Feb 22, 2016 at 10:08:52AM -0700, Jan Beulich wrote:
>> >>> On 22.02.16 at 06:35, <shuai.ruan@xxxxxxxxxxxxxxx> wrote:
>> 
>> First of all I wonder on what basis you collect your Cc lists on
>> patches.
>> 
> I send the bugs-fix patch as whole. I just get the Cc lists using the
> script based on the whole patchset. May be I will send the patch
> seperately.

Thank you. Please also see
http://wiki.xenproject.org/wiki/Submitting_Xen_Project_Patches
and in particular also Konrad's recent suggested adjustments
http://lists.xenproject.org/archives/html/xen-devel/2016-02/msg02877.html
(perhaps including the other parts of the thread).

>> > --- a/xen/arch/x86/i387.c
>> > +++ b/xen/arch/x86/i387.c
>> > @@ -118,7 +118,7 @@ static inline uint64_t vcpu_xsave_mask(const struct 
>> > vcpu *v)
>> >      if ( v->fpu_dirtied )
>> >          return v->arch.nonlazy_xstate_used ? XSTATE_ALL : XSTATE_LAZY;
>> >  
>> > -    return v->arch.nonlazy_xstate_used ? XSTATE_NONLAZY : 0;
>> > +    return ( cpu_has_xsaves || cpu_has_xsavec ) ? XSTATE_ALL : 
>> > XSTATE_NONLAZY;
>> >  }
>> 
>> The description lacks any mention of the performance impact,
>> and what investigation was done to find ways to perhaps
>> overcome this. For example, regardless of cpu_has_xsaves,
>> do we really always need to _use_ XSAVES?
>> 
> Currently no supervisor xstates is enabled in xen or even in
> guest os. Using xsaves is a little ahead (xsavec may used).  
> xsavec may also cause overwriting problem like xsaves.
> I will add performance impact in the description. 
> I am still thinking of a better way to overcome the overhead xsave
> (But have not get a better solution yet).

I was thinking that taking into consideration the features a guest
has ever used (i.e. v->arch.xcr0_accum) to decide which variant
to use may be a worthwhile avenue to explore.

Jan


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