Re: [PATCH] crypto: x86/aes-ni: fix AVX detection

[Paolo Bonzini]

On 11/3/21 15:52, Herbert Xu wrote:
> On Wed, Nov 03, 2021 at 07:43:43AM -0700, Dave Hansen wrote:
> > The kernel shouldn't crash in this case.  We've got a software
> > dependency which should disable AVX2 if AVX is off:
> It's qemu, I thought it was a qemu bug but Paulo disagrees.

I don't disagree that QEMU could do better in preventing nonsensical 
combinations; however, independent of that you shouldn't use AVX2 
without factoring XCR0 in the decision.  This is true no matter if 
userspace/kernel or even bare metal/virt.

That does not have to be done directly with cpu_has_xfeatures as in 
Maxim's patch: if boot_cpu_has(AVX) returns false after 
xsave_cpuid_features has filtered it out, or if boot_cpu_has(AVX2) 
returns false after cpuid_deps has filtered it out, that's fine.  I 
guess Maxim can look at the pointers that Dave provided and check if 
there's another bug somewhere in arch/x86/kernel.

Paolo