Re: [Xen-devel] [PATCH 0/3 v2] XSAVE/XRSTOR fixes and enhancements

[Weidong Han <weidong.han@xxxxxxxxx>]

Keir Fraser wrote:
> On 01/09/2010 02:53, "Weidong Han" <weidong.han@xxxxxxxxx> wrote:
>
>   
> > > Performance overhead of this fix? Is there no other lazy save technique that
> > > can work?
> > >   
> > >       
> > I think the cost of set_xcr0 which just changes some bits in XCR0
> > register should be little.  I don't have any optimization for it now.
> >     
> I obviously don't mean that. What about the increased cost of XSAVE and
> XRSTOR s/r'ing more state unconditionally? At least it is conditional on
> v->fpu_dirtied I suppose?
>   
One possible optimization is that only save/restore legacy states (FPU 
and SSE) for guests which don't enable XSAVE.
Both xsave() and xrstor are invoked only when v->fpu_dirtied.
>   
> > > > Patch 3/3. Enable guest AVX
> > > > This patch enables Intel(R) Advanced Vector Extension (AVX) for guest.
> > > >     
> > > >         
> > > If we enable this but don't implement save/restore then don't guests lose
> > > state across s/r with unpredictable results?
> > >   
> > >       
> > Yes. As I said in another email, actually it already breaks hvm guests
> > save/restore on platforms which supports XSAVE/XRSTOR.
> >     
> Wow, so the last couple of Xen releases are broken for the latest Intel
> platforms unless you specify no-xsave. Handy to know I guess.
>
> Why is the feature flag stuff all stuffed in Xen itself rather than
> xc_cpuid_x86.c, by the way? Shouldn't your change also be in the same place,
> or (much preferably) all XSAVE related stuff be moved out into
> libxc/xc_cpuid_x86.c?
>   
I'm afraid XSAVE related stuff cannot be move out into 
libxc/xc_cpuid_x86.c completely? At least Xen needs to control 
X86_CR4_OSXSAVE for guests which don't support XSAVE. I will have a look 
at it.
Regards,
Weidong
>  -- Keir
>
>   
> > Regards,
> > Weidong
> >     
> > >   
> > >       
>
>   

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