Re: [PATCH v3] lib/raid/xor: x86: Add AVX-512 optimized xor_gen()

[Eric Biggers]

On Mon, Jun 15, 2026 at 09:16:55PM +0000, Borislav Petkov wrote:
> On June 15, 2026 8:10:50 PM UTC, Eric Biggers <ebiggers@xxxxxxxxxx> wrote:
> >
> >But I wanted to ask: do we really care about the case where features are
> >"supported" but their XCR0 bits aren't set?  Perhaps the kernel just
> >doesn't/shouldn't support weird cases like "-cpu max,xsave=off"?
> >
> 
> Yes, our aim is to support only configurations which are actually
> present in real hardware and not a "oh, it would be good if it did
> that, just because..."

Seems reasonable to me.  Would the same apply to UML here?

> >If this case indeed needs to be handled, could we make things easier for
> >the kernel's AVX and AVX-512 optimized code?  Currently AVX-512 needs:
> >
> >        if (boot_cpu_has(X86_FEATURE_AVX512F) &&
> >            cpu_has_xfeatures(XFEATURE_MASK_FP | XFEATURE_MASK_SSE |
> >                              XFEATURE_MASK_YMM | XFEATURE_MASK_AVX512, NULL))
> >
> >How about we make X86_FEATURE_AVX512F depend on XCR0=111xx111, and
> >X86_FEATURE_AVX depend on XCR0=xxxxx111?  Then the cpu_has_xfeatures()
> >check wouldn't be needed.  Is there any reason not to do that?
> 
>  How do you want to accomplish that? Very early during boot on the BSP
>  you sanity-check XCR0 and clear feature flags if components are not
>  set? 

That would be the idea.  Something similar to what
arch/x86/kernel/cpu/cpuid-deps.c does.  Except that seems to only
enforce the dependencies when the kernel itself is disabling things; if
the hypervisor is broken then it just warns.

In any case, I'd like these to go away:

    $ git grep cpu_has_xfeatures | wc -l
    31

- Eric