RE: [PATCH v2 1/5] perf/x86/intel/lbr: use setup_clear_cpu_cap instead of clear_cpu_cap

[H. Peter Anvin]

On November 2, 2022 7:27:52 AM PDT, "Elliott, Robert (Servers)" <elliott@xxxxxxx> wrote:
>
>> From: Paolo Bonzini <pbonzini@xxxxxxxxxx>
>...
>> (2) in particular holds even on bare metal.  The kernel bug here is that
>> X86_FEATURE_AVX only tells you if the instructions are _present_, not if
>> they are _usable_.   Indeed, the XCR0 check is present for all other
>> files in arch/x86/crypto, either instead or in addition to
>> boot_cpu_has(X86_FEATURE_AVX).
>> 
>> Maxim had sent a patch about a year ago to do it in aesni-intel-glue.c
>> but Dave told him to fix the dependencies instead
>> (https://lore.kernel.org/all/20211103124614.499580-1-
>> mlevitsk@xxxxxxxxxx/).
>>   What do you think of applying that patch instead?
>
>Most of the x86 crypto modules using X86_FEATURE_AVX do check
>	cpu_has_xfeatures(XFEATURE_MASK_YMM, ...
>
>so it's probably prudent to add it to the rest (or remove it everywhere
>if it is not needed).
>
>1. Currently checking XSAVE YMM:
>  aria_aesni_avx_glue
>  blake2s-glue
>  camellia_aesni_avx2_glue	camellia_aesni_avx_glue
>  cast5_avx_glue		cast6_avx_glue
>  chacha_glue
>  poly1305_glue
>  serpent_avx2_glue		serpent_avx_glue
>  sha1_ssse3_glue		sha256_ssse3_glue	sha512_ssse3_glue
>  sm3_avx_glue
>  sm4_aesni_avx2_glue	sm4_aesni_avx_glue
>  twofish_avx_glue
>
>Currently not checking XSAVE YMM:
>  aesni-intel_glue
>  curve25519-x86_64
>  nhpoly1305-avx2-glue
>  polyval-clmulni_glue
>
>2. Similarly, modules using X86_FEATURE_AVX512F, X86_FEATURE_AVXX512VL
>and/or X86_FEATURE_AVX512BW probably need to check XFEATURE_MASK_AVX512:
>
>Currently checking XSAVE AVX512:
>  blake2s-glue
>  poly1305_glue
>
>Currently not checking XSAVE AVX512:
>  chacha_glue
>
>3. Similarly, modules using X86_FEATURE_XMM2 probably need to
>check XFEATURE_MASK_SSE:
>
>Currently checking XSAVE SSE:
>  aegis128-aesni-glue 
>
>Current not checking XSAVE SSE:
>  nhpoly1305-sse2_glue
>  serpent_sse2_glue
>
>
>

We have a dependency system for CPUID features. If you are going to do this (as opposed to "fixing" this in Qemu or just saying "don't do that, it isn't a valid hardware configuration."