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

Kevin Hao
2017-01-24T00:15:16Z

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On Mon, Jan 23, 2017 at 01:16:40PM -0800, Yu-cheng Yu wrote:
> On Mon, Jan 23, 2017 at 01:10:20PM -0800, Dave Hansen wrote:
> > The code is:
> >
> > > void fpstate_init(union fpregs_state *state)
> > > {
> > > if (!static_cpu_has(X86_FEATURE_FPU)) {
> > > fpstate_init_soft(&state->soft);
> > > return;
> > > }
> > >
> > > memset(state, 0, fpu_kernel_xstate_size);
> > >
> > > /*
> > > * XRSTORS requires that this bit is set in xcomp_bv, or
> > > * it will #GP. Make sure it is replaced after the memset().
> > > */
> > > if (static_cpu_has(X86_FEATURE_XSAVES))
> > > state->xsave.header.xcomp_bv = XCOMP_BV_COMPACTED_FORMAT;
> >
> > That seems to set it unconditionally. What am I missing?
>
> The fix I am proposing is...
>
> state->xsave.header.xcomp_bv = XCOMP_BV_COMPACTED_FORMAT |
> xfeatures_mask;

Actually I thought about this change before I made this patch, but I don't this
is the right fix. It is always error prone to init the xcomp_bv to all the
supported feature. In case like copyin_to_xsaves(), it is possible that the
features which should be set in xcomp_bv do not equal to all the supported
features. Please see the following codes in copyin_to_xsaves():
/*
* The state that came in from userspace was user-state only.
* Mask all the user states out of 'xfeatures':
*/
xsave->header.xfeatures &= XFEATURE_MASK_SUPERVISOR;

/*
* Add back in the features that came in from userspace:
*/
xsave->header.xfeatures |= xfeatures;

Thanks,
Kevin

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