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

On Mon, Jun 15, 2026 at 02:29:22PM -0700, Eric Biggers wrote:
> On Mon, Jun 15, 2026 at 09:16:55PM +0000, Borislav Petkov wrote:
> > On June 15, 2026 8:10:50 PM UTC, Eric Biggers <[email protected]> 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?

Good question.

Richi?

> > >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. 

Yap.

> Except that seems to only enforce the dependencies when the kernel itself is
> disabling things; if the hypervisor is broken then it just warns.

Not the kernel's problem. We deliberately don't want to maintain a zoo of
options which are not present in real hw. If HV is doing funny things, oh
well...


> In any case, I'd like these to go away:
> 
>     $ git grep cpu_has_xfeatures | wc -l
>     31

Yeah, all in crypto. I can certainly see why.

@dhansen, any other thoughts?

Thx.

-- 
Regards/Gruss,
    Boris.

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