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 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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