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

[Babu Moger]

On 12/7/20 5:06 PM, Jim Mattson wrote:
> On Mon, Dec 7, 2020 at 2:38 PM Babu Moger <babu.moger@xxxxxxx> 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@xxxxxxx>
>> ---
> 
> 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