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

Yu-cheng Yu
2017-01-23T21:20:19Z

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;