linux-kernel - Re: [PATCH 5/6] KVM: x86: Virtualize FLUSH_L1D and passthrough MSR_IA32_FLUSH

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Date:   Mon, 27 Mar 2023 11:33:31 +0800
From:   Xiaoyao Li <xiaoyao.li@...el.com>
To:     Sean Christopherson <seanjc@...gle.com>,
        Paolo Bonzini <pbonzini@...hat.com>
Cc:     kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        Nathan Chancellor <nathan@...nel.org>,
        Emanuele Giuseppe Esposito <eesposit@...hat.com>,
        Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>,
        Jim Mattson <jmattson@...gle.com>
Subject: Re: [PATCH 5/6] KVM: x86: Virtualize FLUSH_L1D and passthrough
 MSR_IA32_FLUSH_CMD

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?