RE: [PATCH v2 2/2] KVM: SVM: Add support for Virtual SPEC_CTRL

[Babu Moger]

> -----Original Message-----
> From: Lendacky, Thomas <Thomas.Lendacky@xxxxxxx>
> Sent: Monday, January 4, 2021 9:47 AM
> To: Moger, Babu <Babu.Moger@xxxxxxx>; pbonzini@xxxxxxxxxx;
> tglx@xxxxxxxxxxxxx; mingo@xxxxxxxxxx; bp@xxxxxxxxx
> Cc: fenghua.yu@xxxxxxxxx; tony.luck@xxxxxxxxx; wanpengli@xxxxxxxxxxx;
> kvm@xxxxxxxxxxxxxxx; peterz@xxxxxxxxxxxxx; seanjc@xxxxxxxxxx;
> joro@xxxxxxxxxx; x86@xxxxxxxxxx; kyung.min.park@xxxxxxxxx; linux-
> kernel@xxxxxxxxxxxxxxx; krish.sadhukhan@xxxxxxxxxx; hpa@xxxxxxxxx;
> mgross@xxxxxxxxxxxxxxx; vkuznets@xxxxxxxxxx; Phillips, Kim
> <kim.phillips@xxxxxxx>; Huang2, Wei <Wei.Huang2@xxxxxxx>;
> jmattson@xxxxxxxxxx
> Subject: Re: [PATCH v2 2/2] KVM: SVM: Add support for Virtual SPEC_CTRL
> 
> On 12/22/20 4:31 PM, Babu Moger wrote:
> > Newer AMD processors have a feature to virtualize the use of the
> > SPEC_CTRL MSR. A hypervisor may wish to impose speculation controls on
> > guest execution or a guest may want to impose its own speculation
> > controls. Therefore, the processor implements both host and guest
> > versions of SPEC_CTRL. Presence of this feature is indicated via CPUID
> > function 0x8000000A_EDX[20]: GuestSpecCtrl.  Hypervisors are not
> > required to enable this feature since it is automatically enabled on
> > processors that support it.
> >
> > When in host mode, the host SPEC_CTRL value is in effect and writes
> > update only the host version of SPEC_CTRL. On a VMRUN, the processor
> > loads the guest version of SPEC_CTRL from the VMCB. When the guest
> > writes SPEC_CTRL, only the guest version is updated. On a VMEXIT, the
> > guest version is saved into the VMCB and the processor returns to only
> > using the host SPEC_CTRL for speculation control. The guest SPEC_CTRL
> > is located at offset 0x2E0 in the VMCB.
> 
> With the SEV-ES hypervisor support now in the tree, this will need to add support
> in sev_es_sync_vmsa() to put the initial svm->spec_ctrl value in the SEV-ES
> VMSA.
> 
> >
> > The effective SPEC_CTRL setting is the guest SPEC_CTRL setting or'ed
> > with the hypervisor SPEC_CTRL setting. This allows the hypervisor to
> > ensure a minimum SPEC_CTRL if desired.
> >
> > This support also fixes an issue where a guest may sometimes see an
> > inconsistent value for the SPEC_CTRL MSR on processors that support
> > this feature. With the current SPEC_CTRL support, the first write to
> > SPEC_CTRL is intercepted and the virtualized version of the SPEC_CTRL
> > MSR is not updated. When the guest reads back the SPEC_CTRL MSR, it
> > will be 0x0, instead of the actual expected value. There isn’t a
> > security concern here, because the host SPEC_CTRL value is or’ed with
> > the Guest SPEC_CTRL value to generate the effective SPEC_CTRL value.
> > KVM writes with the guest's virtualized SPEC_CTRL value to SPEC_CTRL
> > MSR just before the VMRUN, so it will always have the actual value
> > even though it doesn’t appear that way in the guest. The guest will
> > only see the proper value for the SPEC_CTRL register if the guest was
> > to write to the SPEC_CTRL register again. With Virtual SPEC_CTRL
> > support, the MSR interception of SPEC_CTRL is disabled during
> > vmcb_init, so this will no longer be an issue.
> >
> > Signed-off-by: Babu Moger <babu.moger@xxxxxxx>
> > ---
> >   arch/x86/include/asm/svm.h |    4 +++-
> >   arch/x86/kvm/svm/svm.c     |   29 +++++++++++++++++++++++++----
> >   2 files changed, 28 insertions(+), 5 deletions(-)
> >
> > diff --git a/arch/x86/include/asm/svm.h b/arch/x86/include/asm/svm.h
> > index 71d630bb5e08..753b25db427c 100644
> > --- a/arch/x86/include/asm/svm.h
> > +++ b/arch/x86/include/asm/svm.h
> > @@ -248,12 +248,14 @@ struct vmcb_save_area {
> >   	u64 br_to;
> >   	u64 last_excp_from;
> >   	u64 last_excp_to;
> > +	u8 reserved_12[72];
> > +	u32 spec_ctrl;		/* Guest version of SPEC_CTRL at 0x2E0 */
> >
> >   	/*
> >   	 * The following part of the save area is valid only for
> >   	 * SEV-ES guests when referenced through the GHCB.
> >   	 */
> > -	u8 reserved_7[104];
> > +	u8 reserved_7[28];
> >   	u64 reserved_8;		/* rax already available at 0x01f8 */
> >   	u64 rcx;
> >   	u64 rdx;
> > diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c index
> > 79b3a564f1c9..6d3db3e8cdfe 100644
> > --- a/arch/x86/kvm/svm/svm.c
> > +++ b/arch/x86/kvm/svm/svm.c
> > @@ -1230,6 +1230,16 @@ static void init_vmcb(struct vcpu_svm *svm)
> >
> >   	svm_check_invpcid(svm);
> >
> > +	/*
> > +	 * If the host supports V_SPEC_CTRL then disable the interception
> > +	 * of MSR_IA32_SPEC_CTRL.
> > +	 */
> > +	if (boot_cpu_has(X86_FEATURE_V_SPEC_CTRL)) {
> > +		save->spec_ctrl = svm->spec_ctrl;
> > +		set_msr_interception(&svm->vcpu, svm->msrpm,
> > +				     MSR_IA32_SPEC_CTRL, 1, 1);
> > +	}
> > +
> 
> I thought Jim's feedback was to keep the support as originally coded with
> respect to the MSR intercept and only update the svm_vcpu_run() to either
> read/write the MSR or read/write the save area value based on the feature.
> So I think this can be removed.

Ok. Sure. Will remove this change.

> 
> >   	if (kvm_vcpu_apicv_active(&svm->vcpu))
> >   		avic_init_vmcb(svm);
> >
> > @@ -2549,7 +2559,10 @@ static int svm_get_msr(struct kvm_vcpu *vcpu,
> struct msr_data *msr_info)
> >   		    !guest_cpuid_has(vcpu, X86_FEATURE_AMD_SSBD))
> >   			return 1;
> >
> > -		msr_info->data = svm->spec_ctrl;
> > +		if (static_cpu_has(X86_FEATURE_V_SPEC_CTRL))
> > +			msr_info->data = svm->vmcb->save.spec_ctrl;
> > +		else
> > +			msr_info->data = svm->spec_ctrl;
> 
> This is unneeded since svm->vmcb->save.spec_ctrl is saved in
> svm->spec_ctrl on VMEXIT.

Sure.

> 
> >   		break;
> >   	case MSR_AMD64_VIRT_SPEC_CTRL:
> >   		if (!msr_info->host_initiated &&
> > @@ -2640,6 +2653,8 @@ static int svm_set_msr(struct kvm_vcpu *vcpu,
> struct msr_data *msr)
> >   			return 1;
> >
> >   		svm->spec_ctrl = data;
> > +		if (static_cpu_has(X86_FEATURE_V_SPEC_CTRL))
> > +			svm->vmcb->save.spec_ctrl = data;
> 
> And this is unneeded since svm->vmcb->save.spec_ctrl is set to
> svm->spec_ctrl before VMRUN.

Sure.

> 
> >   		if (!data)
> >   			break;
> >
> > @@ -3590,7 +3605,10 @@ static __no_kcsan fastpath_t svm_vcpu_run(struct
> kvm_vcpu *vcpu)
> >   	 * is no need to worry about the conditional branch over the wrmsr
> >   	 * being speculatively taken.
> >   	 */
> > -	x86_spec_ctrl_set_guest(svm->spec_ctrl, svm->virt_spec_ctrl);
> > +	if (static_cpu_has(X86_FEATURE_V_SPEC_CTRL))
> > +		svm->vmcb->save.spec_ctrl = svm->spec_ctrl;
> > +	else
> > +		x86_spec_ctrl_set_guest(svm->spec_ctrl, svm->virt_spec_ctrl);
> >
> >   	svm_vcpu_enter_exit(vcpu, svm);
> >
> > @@ -3609,12 +3627,15 @@ static __no_kcsan fastpath_t
> svm_vcpu_run(struct kvm_vcpu *vcpu)
> >   	 * If the L02 MSR bitmap does not intercept the MSR, then we need to
> >   	 * save it.
> >   	 */
> > -	if (unlikely(!msr_write_intercepted(vcpu, MSR_IA32_SPEC_CTRL)))
> > +	if (static_cpu_has(X86_FEATURE_V_SPEC_CTRL))
> > +		svm->spec_ctrl = svm->vmcb->save.spec_ctrl;
> > +	else if (unlikely(!msr_write_intercepted(vcpu, MSR_IA32_SPEC_CTRL)))
> >   		svm->spec_ctrl = native_read_msr(MSR_IA32_SPEC_CTRL);
> 
> If I understood Jim's feedback correctly, this will change to something like:
> 
> if (unlikely(!msr_write_intercepted(vcpu, MSR_IA32_SPEC_CTRL))) {
> 	if (static_cpu_has(X86_FEATURE_V_SPEC_CTRL))
> 		svm->spec_ctrl = svm->vmcb->save.spec_ctrl;
> 	else
> 		svm->spec_ctrl = native_read_msr(MSR_IA32_SPEC_CTRL);
> }

Sure. Will take care of this in next revision. Thanks
Babu

> 
> Thanks,
> Tom
> 
> >
> >   	reload_tss(vcpu);
> >
> > -	x86_spec_ctrl_restore_host(svm->spec_ctrl, svm->virt_spec_ctrl);
> > +	if (!static_cpu_has(X86_FEATURE_V_SPEC_CTRL))
> > +		x86_spec_ctrl_restore_host(svm->spec_ctrl, svm-
> >virt_spec_ctrl);
> >
> >   	vcpu->arch.cr2 = svm->vmcb->save.cr2;
> >   	vcpu->arch.regs[VCPU_REGS_RAX] = svm->vmcb->save.rax;
> >