From: Babu Moger <babu.moger@amd.com>
To: Jim Mattson <jmattson@google.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>,
Thomas Gleixner <tglx@linutronix.de>,
Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
"Yu, Fenghua" <fenghua.yu@intel.com>,
Tony Luck <tony.luck@intel.com>,
Wanpeng Li <wanpengli@tencent.com>,
kvm list <kvm@vger.kernel.org>,
Tom Lendacky <thomas.lendacky@amd.com>,
Peter Zijlstra <peterz@infradead.org>,
Sean Christopherson <seanjc@google.com>,
Joerg Roedel <joro@8bytes.org>,
the arch/x86 maintainers <x86@kernel.org>,
kyung.min.park@intel.com, LKML <linux-kernel@vger.kernel.org>,
Krish Sadhukhan <krish.sadhukhan@oracle.com>,
"H . Peter Anvin" <hpa@zytor.com>,
mgross@linux.intel.com, Vitaly Kuznetsov <vkuznets@redhat.com>,
kim.phillips@amd.com, wei.huang2@amd.com
Subject: Re: [PATCH 2/2] KVM: SVM: Add support for Virtual SPEC_CTRL
Date: Tue, 22 Dec 2020 10:14:13 -0600 [thread overview]
Message-ID: <491ec523-3076-aa51-c94d-a36ca08f42ca@amd.com> (raw)
In-Reply-To: <CALMp9eRSvWemdiBygMJ18yP9T0UzL0nNbpD__bRis7M5LqOK+g@mail.gmail.com>
On 12/7/20 5:06 PM, Jim Mattson wrote:
> On Mon, Dec 7, 2020 at 2:38 PM Babu Moger <babu.moger@amd.com> wrote:
>>
>> Newer AMD processors have a feature to virtualize the use of the
>> SPEC_CTRL MSR. When supported, the SPEC_CTRL MSR is automatically
>> virtualized and no longer requires hypervisor intervention.
>>
>> This feature is detected 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 this feature is enabled, the hypervisor no longer has to
>> intercept the usage of the SPEC_CTRL MSR and no longer is required to
>> save and restore the guest SPEC_CTRL setting when switching
>> hypervisor/guest modes. 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@amd.com>
>> ---
>
> Shouldn't there be some code to initialize a new "guest SPEC_CTRL"
> value in the VMCB, both at vCPU creation, and at virtual processor
> reset?
>
>> arch/x86/kvm/svm/svm.c | 17 ++++++++++++++---
>> 1 file changed, 14 insertions(+), 3 deletions(-)
>>
>> diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
>> index 79b3a564f1c9..3d73ec0cdb87 100644
>> --- a/arch/x86/kvm/svm/svm.c
>> +++ b/arch/x86/kvm/svm/svm.c
>> @@ -1230,6 +1230,14 @@ 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))
>> + set_msr_interception(&svm->vcpu, svm->msrpm, MSR_IA32_SPEC_CTRL,
>> + 1, 1);
>> +
>> if (kvm_vcpu_apicv_active(&svm->vcpu))
>> avic_init_vmcb(svm);
>>
>> @@ -3590,7 +3598,8 @@ 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))
>> + x86_spec_ctrl_set_guest(svm->spec_ctrl, svm->virt_spec_ctrl);
>
> Is this correct for the nested case? Presumably, there is now a "guest
> SPEC_CTRL" value somewhere in the VMCB. If L1 does not intercept this
> MSR, then we need to transfer the "guest SPEC_CTRL" value from the
> vmcb01 to the vmcb02, don't we?
>
>> svm_vcpu_enter_exit(vcpu, svm);
>>
>> @@ -3609,12 +3618,14 @@ 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) &&
>> + unlikely(!msr_write_intercepted(vcpu, MSR_IA32_SPEC_CTRL)))
>> svm->spec_ctrl = native_read_msr(MSR_IA32_SPEC_CTRL);
>
> Is this correct for the nested case? If L1 does not intercept this
> MSR, then it might have changed while L2 is running. Presumably, the
> hardware has stored the new value somewhere in the vmcb02 at #VMEXIT,
> but now we need to move that value into the vmcb01, don't we?
>
>> 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;
>>
>
> It would be great if you could add some tests to kvm-unit-tests.
>
Posted the kvm unit tests. Let me know the feedback.
https://lore.kernel.org/kvm/160865324865.19910.5159218511905134908.stgit@bmoger-ubuntu/
Thanks
Babu
next prev parent reply other threads:[~2020-12-22 16:15 UTC|newest]
Thread overview: 16+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-12-07 22:37 [PATCH 0/2] x86: Add the feature Virtual SPEC_CTRL Babu Moger
2020-12-07 22:37 ` [PATCH 1/2] x86/cpufeatures: Add the Virtual SPEC_CTRL feature Babu Moger
2020-12-07 23:22 ` Jim Mattson
2020-12-09 22:39 ` Babu Moger
2020-12-09 23:11 ` Jim Mattson
2020-12-22 16:14 ` Babu Moger
2020-12-22 17:41 ` Sean Christopherson
2020-12-22 18:07 ` Babu Moger
2020-12-07 22:37 ` [PATCH 2/2] KVM: SVM: Add support for Virtual SPEC_CTRL Babu Moger
2020-12-07 23:06 ` Jim Mattson
2020-12-10 21:26 ` Babu Moger
2020-12-10 21:36 ` Jim Mattson
2020-12-10 22:57 ` Babu Moger
2020-12-22 16:14 ` Babu Moger [this message]
2020-12-07 23:13 ` Sean Christopherson
2020-12-10 21:31 ` Babu Moger
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