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

[Shuai Ruan <shuai.ruan@xxxxxxxxxxxxxxx>]

On Wed, Feb 24, 2016 at 02:16:38AM -0700, Jan Beulich wrote:
> > 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).
Thanks.
> 
> >> > --- 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.
> 
Oh, Thanks for your suggestion.
You mean when (v->arch.xcr0_accum & (XSTATE_LAZY & ~XSTATE_FP_SSE)) return 
false,
we can return XSTATE_NONLAZY in vcpu_xsave_mask when using xsave[cs]
otherwise return XSTATE_ALL.
It means we can save the area safely, if the guest has ever use 
XSTATE_NONLAZY | XSTATE_FP_SSE only. 
> Jan
> 
> 
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