Re: [PATCH v7 02/26] x86/fpu/xstate: Refine CET user xstate bit enabling

[Yang, Weijiang]

On 12/14/2023 1:01 AM, Chang S. Bae wrote:
> On 12/13/2023 1:30 AM, Yang, Weijiang wrote:
> > Let me involve Chang, the author of the code in question.
> >
> > Hi, Chang,
> > In commit 70c3f1671b0c ("x86/fpu/xstate: Prepare XSAVE feature table for gaps in state component numbers"),
> > you modified the loop as below:
> >          for (i = 0; i < ARRAY_SIZE(xsave_cpuid_features); i++) {
> > -               if (!boot_cpu_has(xsave_cpuid_features[i]))
> > +               unsigned short cid = xsave_cpuid_features[i];
> > +
> > +               /* Careful: X86_FEATURE_FPU is 0! */
> > +               if ((i != XFEATURE_FP && !cid) || !boot_cpu_has(cid))
> >                          fpu_kernel_cfg.max_features &= ~BIT_ULL(i);
> >          }
> >
> > IMHO the change resulted functional change of the loop, i.e., before that only it only clears the bits without CPUIDs,
> > but after the change, the side-effect of the loop will clear the bits of blank entries ( where xsave_cpuid_features[i] == 0 )
> > since the loop hits (i != XFEATURE_FP && !cid), is it intended or something else?
>
> The code was considered as much *simpler* than the other [1]. Yes, it clears those not listed in the table but they were either non-existed or disabled at that moment.

Thanks Chang, so I assume the "side-effect" is intended.

> Thanks,
> Chang
>
> [1] https://lore.kernel.org/lkml/87y2eha576.fsf@xxxxxxxxxxxxxxxxxxxxxxx/