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

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Message-ID: <ZCG9hdSHvM56z/FZ@google.com>
Date:   Mon, 27 Mar 2023 09:00:05 -0700
From:   Sean Christopherson <seanjc@...gle.com>
To:     Jim Mattson <jmattson@...gle.com>
Cc:     Xiaoyao Li <xiaoyao.li@...el.com>,
        Paolo Bonzini <pbonzini@...hat.com>, 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>
Subject: Re: [PATCH 5/6] KVM: x86: Virtualize FLUSH_L1D and passthrough MSR_IA32_FLUSH_CMD

On Mon, Mar 27, 2023, Jim Mattson wrote:
> On Sun, Mar 26, 2023 at 8:33 PM Xiaoyao Li <xiaoyao.li@...el.com> wrote:
> >
> > 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?
> 
> The L1D$ isn't very big. A guest could always flush out any previously
> cached data simply by referencing its own data. Is the ability to
> flush the L1D$ by WRMSR that egregious?

Yeah, AFAIK RDT and the like only provide QoS controls for L3, so L1 is fair game.