Re: [tip:x86/urgent] x86/fpu: Set the xcomp_bv when we fake up a XSAVES area

[Kevin Hao]

On Mon, Jan 23, 2017 at 06:01:10PM -0800, Dave Hansen wrote:
> On 01/23/2017 05:50 PM, Kevin Hao wrote:
> > According to the ISA manual, XSAVES also set the XCOMP_BV[62:0]. My code only
> > try to be compatible with what the cpu does when excuting XSAVES. The following
> > is quoted from 325462-sdm-vol-1-2abcd-3abcd.pdf.
> >   The XSAVES instructions sets bit 63 of the XCOMP_BV field of the XSAVE header while writing RFBM[62:0] to
> >   XCOMP_BV[62:0]. The XSAVES instruction does not write any part of the XSAVE header other than the XSTATE_BV
> >   and XCOMP_BV fields.
> 
> What purpose does it serve to make copyin_to_xsaves() set that bit,

We try to fake up a memory area which is supposed to be composed by XSAVES
instruction. My code is just trying to do what the XSAVES do.

> other than helping to hide bugs?

Why do you think it hide the bug? In contrast, I think my patch fixes what the
bug really is. The memory area we fake up is bug, we should fix it there.

Thanks,
Kevin

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