Re: [Xen-devel] [V4] x86/xsaves: fix overwriting between non-lazy/lazy xsaves

[Shuai Ruan <shuai.ruan@xxxxxxxxxxxxxxx>]

On Tue, Mar 15, 2016 at 07:33:40AM -0600, Jan Beulich wrote:
> >>> On 15.03.16 at 10:40, <shuai.ruan@xxxxxxxxxxxxxxx> wrote:
> >     xrstor() will look as follow:
> >     if ( using_xsaves )
> >     {
> >             if ( unlikely(!(prt->xsave_hdr->xcom_bv &
> >                           XSTATE_COMPACTION_ENABLED)) )
> >                    ptr->xsave_hdr->xcomp_bv =
> >                                 ptr->xsave_hdr->xstate_bv |
> >                                 XSTATE_COMPACTION_ENABLED;
> >             XRSTORS;
> >     }
> >     else
> >             XRSTOR;
> 
> This makes me imply that "using_xsaves" is still a global variable,
> set depending on CPU features. That's exactly what I've said
> would presumably not be sufficient in code like xrstor(). What
> point is there in using XSTORS if the guest never touched XSS?
> I would much rather have expected for you to introduce a
> flag paralleling v->arch.nonlazy_xstate_used indicating
> whether for a particular vCPU XSAVES/XRSTORS really need to be
> used (or maybe just looking at xcr0_accum would be sufficient,
> and no new flag is needed; in fact I think that flag would also
> better go away in favor of just inspecting xcr0_accum).

if xrstor() side depend on checking xcr0_accum and using_xsaves,
then xsave() can not only depend on using_xsaves (or
X86_FEATURE_USE_XSAVES). So I will drop alternativer patching 
in xsave() side.
And both xsave() xrestor() will depend on using_xsaves and checking 
xcr0_accum. And compress_xsave_states() will check this too.

Detail is :

#define XSTATE_SUPER 0
#define using_xsaves 0

if ( using_xsaves && (v->arch.xcr0_accum & XSTATE_SUPER) )
{

.....
        XSAVES/XRSTORS;
}

....

Thanks
> 
> Jan
> 

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