Re: [PATCH v3 1/7] x86: KVM: Move existing x86 CPUID leaf [CPUID_7_1_EAX] to kvm-only leaf

Xiaoyao Li
2022-11-15T13:52:42Z

On 11/10/2022 9:52 AM, Jiaxi Chen wrote:
> cpuid_leaf[12] CPUID_7_1_EAX has only two bits are in use currently:
>
> - AVX-VNNI CPUID.(EAX=7,ECX=1):EAX[bit 4]
> - AVX512-BF16 CPUID.(EAX=7,ECX=1):EAX[bit 5]
>
> These two bits have no other kernel usages other than the guest
> CPUID advertisement in KVM. Given that and to save space for kernel
> feature bits, move these two bits to the kvm-only subleaves. The
> existing leaf cpuid_leafs[12] is set to CPUID_LNX_5 so future feature
> can pick it. This basically reverts:
>
> - commit b85a0425d805 ("Enumerate AVX Vector Neural Network
> instructions")
> - commit b302e4b176d0 ("x86/cpufeatures: Enumerate the new AVX512
> BFLOAT16 instructions")
> - commit 1085a6b585d7 ("KVM: Expose AVX_VNNI instruction to guset")

FYI, LAM support has been queued in tip
https://git.kernel.org/pub/scm/linux/kernel/git/tip/tip.git/commit/?id=aa387b1b1e666cacffc0b7ac7e0949a68013b2d9

It adds

+#define X86_FEATURE_LAM (12*32+26) /* Linear Address Masking */

and conflict with this patch.

Seen from the ISE, there are more bits defined in CPUID_7_1_EAX. And I
believe Intel will define more and it's likely some of them will be used
by kernel just like LAM.

> Signed-off-by: Jiaxi Chen <jiaxi.chen@linux.intel.com>
> ---
> arch/x86/include/asm/cpufeature.h | 2 +-
> arch/x86/include/asm/cpufeatures.h | 4 +---
> arch/x86/kernel/cpu/common.c | 6 ------
> arch/x86/kernel/cpu/cpuid-deps.c | 1 -
> arch/x86/kvm/cpuid.c | 2 +-
> arch/x86/kvm/reverse_cpuid.h | 5 +++++
> 6 files changed, 8 insertions(+), 12 deletions(-)
>
> diff --git a/arch/x86/include/asm/cpufeature.h b/arch/x86/include/asm/cpufeature.h
> index 1a85e1fb0922..b2905ddd7ab4 100644
> --- a/arch/x86/include/asm/cpufeature.h
> +++ b/arch/x86/include/asm/cpufeature.h
> @@ -24,7 +24,7 @@ enum cpuid_leafs
> CPUID_7_0_EBX,
> CPUID_D_1_EAX,
> CPUID_LNX_4,
> - CPUID_7_1_EAX,
> + CPUID_LNX_5,
> CPUID_8000_0008_EBX,
> CPUID_6_EAX,
> CPUID_8000_000A_EDX,
> diff --git a/arch/x86/include/asm/cpufeatures.h b/arch/x86/include/asm/cpufeatures.h
> index b71f4f2ecdd5..ec468d6eaf06 100644
> --- a/arch/x86/include/asm/cpufeatures.h
> +++ b/arch/x86/include/asm/cpufeatures.h
> @@ -305,9 +305,7 @@
> #define X86_FEATURE_USE_IBPB_FW (11*32+16) /* "" Use IBPB during runtime firmware calls */
> #define X86_FEATURE_RSB_VMEXIT_LITE (11*32+17) /* "" Fill RSB on VM exit when EIBRS is enabled */
>
> -/* Intel-defined CPU features, CPUID level 0x00000007:1 (EAX), word 12 */
> -#define X86_FEATURE_AVX_VNNI (12*32+ 4) /* AVX VNNI instructions */
> -#define X86_FEATURE_AVX512_BF16 (12*32+ 5) /* AVX512 BFLOAT16 instructions */
> +/* Linux-defined mapping, word 12 */
>
> /* AMD-defined CPU features, CPUID level 0x80000008 (EBX), word 13 */
> #define X86_FEATURE_CLZERO (13*32+ 0) /* CLZERO instruction */
> diff --git a/arch/x86/kernel/cpu/common.c b/arch/x86/kernel/cpu/common.c
> index 3e508f239098..0c19c84d7ba0 100644
> --- a/arch/x86/kernel/cpu/common.c
> +++ b/arch/x86/kernel/cpu/common.c
> @@ -1026,12 +1026,6 @@ void get_cpu_cap(struct cpuinfo_x86 *c)
> c->x86_capability[CPUID_7_0_EBX] = ebx;
> c->x86_capability[CPUID_7_ECX] = ecx;
> c->x86_capability[CPUID_7_EDX] = edx;
> -
> - /* Check valid sub-leaf index before accessing it */
> - if (eax >= 1) {
> - cpuid_count(0x00000007, 1, &eax, &ebx, &ecx, &edx);
> - c->x86_capability[CPUID_7_1_EAX] = eax;
> - }
> }
>
> /* Extended state features: level 0x0000000d */
> diff --git a/arch/x86/kernel/cpu/cpuid-deps.c b/arch/x86/kernel/cpu/cpuid-deps.c
> index c881bcafba7d..a88e0e8c39fa 100644
> --- a/arch/x86/kernel/cpu/cpuid-deps.c
> +++ b/arch/x86/kernel/cpu/cpuid-deps.c
> @@ -68,7 +68,6 @@ static const struct cpuid_dep cpuid_deps[] = {
> { X86_FEATURE_CQM_OCCUP_LLC, X86_FEATURE_CQM_LLC },
> { X86_FEATURE_CQM_MBM_TOTAL, X86_FEATURE_CQM_LLC },
> { X86_FEATURE_CQM_MBM_LOCAL, X86_FEATURE_CQM_LLC },
> - { X86_FEATURE_AVX512_BF16, X86_FEATURE_AVX512VL },
> { X86_FEATURE_AVX512_FP16, X86_FEATURE_AVX512BW },
> { X86_FEATURE_ENQCMD, X86_FEATURE_XSAVES },
> { X86_FEATURE_PER_THREAD_MBA, X86_FEATURE_MBA },
> diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
> index 7065462378e2..89f5e7f0402b 100644
> --- a/arch/x86/kvm/cpuid.c
> +++ b/arch/x86/kvm/cpuid.c
> @@ -656,7 +656,7 @@ void kvm_set_cpu_caps(void)
> if (boot_cpu_has(X86_FEATURE_AMD_SSBD))
> kvm_cpu_cap_set(X86_FEATURE_SPEC_CTRL_SSBD);
>
> - kvm_cpu_cap_mask(CPUID_7_1_EAX,
> + kvm_cpu_cap_init_scattered(CPUID_7_1_EAX,
> F(AVX_VNNI) | F(AVX512_BF16)
> );
>
> diff --git a/arch/x86/kvm/reverse_cpuid.h b/arch/x86/kvm/reverse_cpuid.h
> index a19d473d0184..674de5b24f8d 100644
> --- a/arch/x86/kvm/reverse_cpuid.h
> +++ b/arch/x86/kvm/reverse_cpuid.h
> @@ -13,6 +13,7 @@
> */
> enum kvm_only_cpuid_leafs {
> CPUID_12_EAX = NCAPINTS,
> + CPUID_7_1_EAX,
> NR_KVM_CPU_CAPS,
>
> NKVMCAPINTS = NR_KVM_CPU_CAPS - NCAPINTS,
> @@ -24,6 +25,10 @@ enum kvm_only_cpuid_leafs {
> #define KVM_X86_FEATURE_SGX1 KVM_X86_FEATURE(CPUID_12_EAX, 0)
> #define KVM_X86_FEATURE_SGX2 KVM_X86_FEATURE(CPUID_12_EAX, 1)
>
> +/* Intel-defined sub-features, CPUID level 0x00000007:1 (EAX) */
> +#define X86_FEATURE_AVX_VNNI KVM_X86_FEATURE(CPUID_7_1_EAX, 4)
> +#define X86_FEATURE_AVX512_BF16 KVM_X86_FEATURE(CPUID_7_1_EAX, 5)
> +
> struct cpuid_reg {
> u32 function;
> u32 index;