Re: [PATCH 5/6] KVM: x86: Virtualize FLUSH_L1D and passthrough MSR_IA32_FLUSH_CMD

[Pawan Gupta <pawan.kumar.gupta@linux.intel.com>]

On Tue, Mar 21, 2023 at 06:14:39PM -0700, Sean Christopherson wrote:
> Virtualize FLUSH_L1D so that the guest can use the performant L1D flush
> if one of the many mitigations might require a flush in the guest, e.g.
> Linux provides an option to flush the L1D when switching mms.
> 
> Passthrough MSR_IA32_FLUSH_CMD for write when it's supported in hardware
> and exposed to the guest, i.e. always let the guest write it directly if
> FLUSH_L1D is fully supported.
> 
> Forward writes to hardware in host context on the off chance that KVM
> ends up emulating a WRMSR, or in the really unlikely scenario where
> userspace wants to force a flush.  Restrict these forwarded WRMSRs to
> the known command out of an abundance of caution.  Passing through the
> MSR means the guest can throw any and all values at hardware, but doing
> so in host context is arguably a bit more dangerous.
> 
> Link: https://lkml.kernel.org/r/CALMp9eTt3xzAEoQ038bJQ9LN0ZOXrSWsN7xnNUD%2B0SS%3DWwF7Pg%40mail.gmail.com
> Link: https://lore.kernel.org/all/20230201132905.549148-2-eesposit@redhat.com
> Signed-off-by: Sean Christopherson <seanjc@google.com>
> ---
>  arch/x86/kvm/cpuid.c      |  2 +-
>  arch/x86/kvm/svm/svm.c    |  5 +++++
>  arch/x86/kvm/vmx/nested.c |  3 +++
>  arch/x86/kvm/vmx/vmx.c    |  5 +++++
>  arch/x86/kvm/vmx/vmx.h    |  2 +-
>  arch/x86/kvm/x86.c        | 12 ++++++++++++
>  6 files changed, 27 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
> index 599aebec2d52..9583a110cf5f 100644
> --- a/arch/x86/kvm/cpuid.c
> +++ b/arch/x86/kvm/cpuid.c
> @@ -653,7 +653,7 @@ void kvm_set_cpu_caps(void)
>  		F(SPEC_CTRL_SSBD) | F(ARCH_CAPABILITIES) | F(INTEL_STIBP) |
>  		F(MD_CLEAR) | F(AVX512_VP2INTERSECT) | F(FSRM) |
>  		F(SERIALIZE) | F(TSXLDTRK) | F(AVX512_FP16) |
> -		F(AMX_TILE) | F(AMX_INT8) | F(AMX_BF16)
> +		F(AMX_TILE) | F(AMX_INT8) | F(AMX_BF16) | F(FLUSH_L1D)
>  	);
>  
>  	/* TSC_ADJUST and ARCH_CAPABILITIES are emulated in software. */
> diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
> index 85bb535fc321..b32edaf5a74b 100644
> --- a/arch/x86/kvm/svm/svm.c
> +++ b/arch/x86/kvm/svm/svm.c
> @@ -95,6 +95,7 @@ static const struct svm_direct_access_msrs {
>  #endif
>  	{ .index = MSR_IA32_SPEC_CTRL,			.always = false },
>  	{ .index = MSR_IA32_PRED_CMD,			.always = false },
> +	{ .index = MSR_IA32_FLUSH_CMD,			.always = false },
>  	{ .index = MSR_IA32_LASTBRANCHFROMIP,		.always = false },
>  	{ .index = MSR_IA32_LASTBRANCHTOIP,		.always = false },
>  	{ .index = MSR_IA32_LASTINTFROMIP,		.always = false },
> @@ -4140,6 +4141,10 @@ static void svm_vcpu_after_set_cpuid(struct kvm_vcpu *vcpu)
>  		set_msr_interception(vcpu, svm->msrpm, MSR_IA32_PRED_CMD, 0,
>  				     !!guest_has_pred_cmd_msr(vcpu));
>  
> +	if (boot_cpu_has(X86_FEATURE_FLUSH_L1D))

Just curious, will this ever be true on AMD hardware? AFAIK,
X86_FEATURE_FLUSH_L1D is Intel-defined CPU feature.

> +		set_msr_interception(vcpu, svm->msrpm, MSR_IA32_FLUSH_CMD, 0,
> +				     !!guest_cpuid_has(vcpu, X86_FEATURE_FLUSH_L1D));
> +
>  	/* For sev guests, the memory encryption bit is not reserved in CR3.  */
>  	if (sev_guest(vcpu->kvm)) {
>  		best = kvm_find_cpuid_entry(vcpu, 0x8000001F);