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Date: Mon, 4 Jan 2021 12:35:33 -0600
From: Babu Moger <babu.moger@....com>
To: "Lendacky, Thomas" <Thomas.Lendacky@....com>,
"pbonzini@...hat.com" <pbonzini@...hat.com>,
"tglx@...utronix.de" <tglx@...utronix.de>,
"mingo@...hat.com" <mingo@...hat.com>,
"bp@...en8.de" <bp@...en8.de>
Cc: "fenghua.yu@...el.com" <fenghua.yu@...el.com>,
"tony.luck@...el.com" <tony.luck@...el.com>,
"wanpengli@...cent.com" <wanpengli@...cent.com>,
"kvm@...r.kernel.org" <kvm@...r.kernel.org>,
"peterz@...radead.org" <peterz@...radead.org>,
"seanjc@...gle.com" <seanjc@...gle.com>,
"joro@...tes.org" <joro@...tes.org>,
"x86@...nel.org" <x86@...nel.org>,
"kyung.min.park@...el.com" <kyung.min.park@...el.com>,
"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
"krish.sadhukhan@...cle.com" <krish.sadhukhan@...cle.com>,
"hpa@...or.com" <hpa@...or.com>,
"mgross@...ux.intel.com" <mgross@...ux.intel.com>,
"vkuznets@...hat.com" <vkuznets@...hat.com>,
"Phillips, Kim" <kim.phillips@....com>,
"Huang2, Wei" <Wei.Huang2@....com>,
"jmattson@...gle.com" <jmattson@...gle.com>
Subject: RE: [PATCH v2 2/2] KVM: SVM: Add support for Virtual SPEC_CTRL
> -----Original Message-----
> From: Lendacky, Thomas <Thomas.Lendacky@....com>
> Sent: Monday, January 4, 2021 9:47 AM
> To: Moger, Babu <Babu.Moger@....com>; pbonzini@...hat.com;
> tglx@...utronix.de; mingo@...hat.com; bp@...en8.de
> Cc: fenghua.yu@...el.com; tony.luck@...el.com; wanpengli@...cent.com;
> kvm@...r.kernel.org; peterz@...radead.org; seanjc@...gle.com;
> joro@...tes.org; x86@...nel.org; kyung.min.park@...el.com; linux-
> kernel@...r.kernel.org; krish.sadhukhan@...cle.com; hpa@...or.com;
> mgross@...ux.intel.com; vkuznets@...hat.com; Phillips, Kim
> <kim.phillips@....com>; Huang2, Wei <Wei.Huang2@....com>;
> jmattson@...gle.com
> 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@....com>
> > ---
> > 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;
> >
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