Re: [RFC PATCH v2 02/11] KVM: x86: Advertise CPUID.7.2.EDX and RRSBA_CTRL support

[Binbin Wu]

On 4/14/2023 2:25 PM, Chao Gao wrote:
> From: Zhang Chen <chen.zhang@xxxxxxxxx>
>
> Add a kvm-only CPUID feature leaf for CPUID.7.2.EDX and RRSBA_CTRL
> as the first feature in the leaf.
>
> RRSBA_CTRL is enumerated by CPUID.7.2.EDX[2]. If supported, RRSBA_DIS_U
> (bit 5) and RRSBA_DIS_S (bit 6) of IA32_SPEC_CTRL MSR can be used to
> disable RRSBA behavior for CPL3 and CPL0/1/2 respectively.
>
> Note that KVM does not intercept guests' IA32_SPEC_CTRL MSR accesses
> after a non-zero is written to the MSR. Therefore, guests can already
> toggle the two bits if the host supports RRSBA_CTRL, and no extra code
> is needed to allow guests to toggle the two bits.
>
> Signed-off-by: Zhang Chen <chen.zhang@xxxxxxxxx>
> Signed-off-by: Chao Gao <chao.gao@xxxxxxxxx>
> Tested-by: Jiaan Lu <jiaan.lu@xxxxxxxxx>
> ---
>   arch/x86/kvm/cpuid.c         | 22 +++++++++++++++++++---
>   arch/x86/kvm/reverse_cpuid.h |  7 +++++++
>   2 files changed, 26 insertions(+), 3 deletions(-)
>
> diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
> index 9583a110cf5f..f024c3ac2203 100644
> --- a/arch/x86/kvm/cpuid.c
> +++ b/arch/x86/kvm/cpuid.c
> @@ -685,6 +685,10 @@ void kvm_set_cpu_caps(void)
>   		SF(SGX1) | SF(SGX2) | SF(SGX_EDECCSSA)
>   	);
>   
> +	kvm_cpu_cap_init_kvm_defined(CPUID_7_2_EDX,
> +		SF(RRSBA_CTRL)
> +	);
> +

Is it slightly better to put the new added one after

    kvm_cpu_cap_init_kvm_defined(CPUID_7_1_EDX, ...

to make it in order?


>   	kvm_cpu_cap_mask(CPUID_8000_0001_ECX,
>   		F(LAHF_LM) | F(CMP_LEGACY) | 0 /*SVM*/ | 0 /* ExtApicSpace */ |
>   		F(CR8_LEGACY) | F(ABM) | F(SSE4A) | F(MISALIGNSSE) |
> @@ -949,13 +953,14 @@ static inline int __do_cpuid_func(struct kvm_cpuid_array *array, u32 function)
>   		break;
>   	/* function 7 has additional index. */
>   	case 7:
> -		entry->eax = min(entry->eax, 1u);
> +		entry->eax = min(entry->eax, 2u);
>   		cpuid_entry_override(entry, CPUID_7_0_EBX);
>   		cpuid_entry_override(entry, CPUID_7_ECX);
>   		cpuid_entry_override(entry, CPUID_7_EDX);
>   
> -		/* KVM only supports 0x7.0 and 0x7.1, capped above via min(). */
> -		if (entry->eax == 1) {
> +		max_idx = entry->eax;
> +
> +		if (max_idx >= 1) {
>   			entry = do_host_cpuid(array, function, 1);
>   			if (!entry)
>   				goto out;
> @@ -965,6 +970,17 @@ static inline int __do_cpuid_func(struct kvm_cpuid_array *array, u32 function)
>   			entry->ebx = 0;
>   			entry->ecx = 0;
>   		}
> +
> +		if (max_idx >= 2) {
> +			entry = do_host_cpuid(array, function, 2);
> +			if (!entry)
> +				goto out;
> +
> +			cpuid_entry_override(entry, CPUID_7_2_EDX);
> +			entry->eax = 0;
> +			entry->ebx = 0;
> +			entry->ecx = 0;
> +		}
>   		break;
>   	case 0xa: { /* Architectural Performance Monitoring */
>   		union cpuid10_eax eax;
> diff --git a/arch/x86/kvm/reverse_cpuid.h b/arch/x86/kvm/reverse_cpuid.h
> index a5717282bb9c..72bad8314a9c 100644
> --- a/arch/x86/kvm/reverse_cpuid.h
> +++ b/arch/x86/kvm/reverse_cpuid.h
> @@ -15,6 +15,7 @@ enum kvm_only_cpuid_leafs {
>   	CPUID_12_EAX	 = NCAPINTS,
>   	CPUID_7_1_EDX,
>   	CPUID_8000_0007_EDX,
> +	CPUID_7_2_EDX,
>   	NR_KVM_CPU_CAPS,
>   
>   	NKVMCAPINTS = NR_KVM_CPU_CAPS - NCAPINTS,
> @@ -47,6 +48,9 @@ enum kvm_only_cpuid_leafs {
>   /* CPUID level 0x80000007 (EDX). */
>   #define KVM_X86_FEATURE_CONSTANT_TSC	KVM_X86_FEATURE(CPUID_8000_0007_EDX, 8)
>   
> +/* Intel-defined sub-features, CPUID level 0x00000007:2 (EDX) */
> +#define KVM_X86_FEATURE_RRSBA_CTRL	KVM_X86_FEATURE(CPUID_7_2_EDX, 2)
> +
>   struct cpuid_reg {
>   	u32 function;
>   	u32 index;
> @@ -69,6 +73,7 @@ static const struct cpuid_reg reverse_cpuid[] = {
>   	[CPUID_8000_0007_EBX] = {0x80000007, 0, CPUID_EBX},
>   	[CPUID_7_EDX]         = {         7, 0, CPUID_EDX},
>   	[CPUID_7_1_EAX]       = {         7, 1, CPUID_EAX},
> +	[CPUID_7_2_EDX]       = {         7, 2, CPUID_EDX},
>   	[CPUID_12_EAX]        = {0x00000012, 0, CPUID_EAX},
>   	[CPUID_8000_001F_EAX] = {0x8000001f, 0, CPUID_EAX},
>   	[CPUID_7_1_EDX]       = {         7, 1, CPUID_EDX},
> @@ -108,6 +113,8 @@ static __always_inline u32 __feature_translate(int x86_feature)
>   		return KVM_X86_FEATURE_SGX_EDECCSSA;
>   	else if (x86_feature == X86_FEATURE_CONSTANT_TSC)
>   		return KVM_X86_FEATURE_CONSTANT_TSC;
> +	else if (x86_feature == X86_FEATURE_RRSBA_CTRL)
> +		return KVM_X86_FEATURE_RRSBA_CTRL;
>   
>   	return x86_feature;
>   }