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

Xiaoyao Li
2023-03-27T03:34:28Z

On 3/22/2023 9:14 AM, Sean Christopherson wrote:
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index c83ec88da043..3c58dbae7b4c 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -3628,6 +3628,18 @@ int kvm_set_msr_common(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
>
> wrmsrl(MSR_IA32_PRED_CMD, PRED_CMD_IBPB);
> break;
> + case MSR_IA32_FLUSH_CMD:
> + if (!msr_info->host_initiated &&
> + !guest_cpuid_has(vcpu, X86_FEATURE_FLUSH_L1D))
> + return 1;
> +
> + if (!boot_cpu_has(X86_FEATURE_FLUSH_L1D) || (data & ~L1D_FLUSH))
> + return 1;
> + if (!data)
> + break;
> +
> + wrmsrl(MSR_IA32_FLUSH_CMD, L1D_FLUSH);
> + break;

Then KVM provides the ability to flush the L1 data cache of host to
userspace. Can it be exploited to degrade the host performance if
userspace VMM keeps flushing the L1 data cache?