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