Re: [RFC PATCH v2 03/11] KVM: x86: Advertise BHI_CTRL support

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On 4/14/2023 2:25 PM, Chao Gao wrote:
From: Zhang Chen <chen.zhang@xxxxxxxxx>

Add 100% kvm-only feature for BHI_CTRL because the kernel doesn't use it
at all.

BHI_CTRL is enumerated by CPUID.7.2.EDX[4]. If supported, BHI_DIS_S (bit
10) of IA32_SPEC_CTRL MSR can be used to enable BHI_DIS_S behavior.

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 BHI_DIS_S bit if the host supports BHI_CTRL, and no extra
code is needed to allow guests to toggle the bit.

Same as Patch 2, please first fix virtualization of MSR_IA32_SPEC_CTRL. Otherwise the this patch makes no sense. E.g, if only X86_FEATURE_BHI_CTRL is exposed to guest without any other CPUID bits related to MSR_IA32_SPEC_CTRL, then guest cannot write MSR_IA32_SPEC_CTRL at all.

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         | 2 +-
  arch/x86/kvm/reverse_cpuid.h | 1 +
  2 files changed, 2 insertions(+), 1 deletion(-)

diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
index f024c3ac2203..7cdd859d09a2 100644
--- a/arch/x86/kvm/cpuid.c
+++ b/arch/x86/kvm/cpuid.c
@@ -686,7 +686,7 @@ void kvm_set_cpu_caps(void)
  	);
kvm_cpu_cap_init_kvm_defined(CPUID_7_2_EDX,
-		SF(RRSBA_CTRL)
+		SF(RRSBA_CTRL) | F(BHI_CTRL)
  	);
kvm_cpu_cap_mask(CPUID_8000_0001_ECX,
diff --git a/arch/x86/kvm/reverse_cpuid.h b/arch/x86/kvm/reverse_cpuid.h
index 72bad8314a9c..e7e70c9aa384 100644
--- a/arch/x86/kvm/reverse_cpuid.h
+++ b/arch/x86/kvm/reverse_cpuid.h
@@ -50,6 +50,7 @@ enum kvm_only_cpuid_leafs {
/* Intel-defined sub-features, CPUID level 0x00000007:2 (EDX) */
  #define KVM_X86_FEATURE_RRSBA_CTRL	KVM_X86_FEATURE(CPUID_7_2_EDX, 2)
+#define X86_FEATURE_BHI_CTRL		KVM_X86_FEATURE(CPUID_7_2_EDX, 4)
struct cpuid_reg {
  	u32 function;




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