Re: [9/8] KVM: x86: limit MSR_IA32_SPEC_CTRL access based on CPUID availability

[Mihai Carabas <mihai.carabas@oracle.com>]

On 30.01.2018 18:33, Jim Mattson wrote:
> All MSR intercepts are enabled by default, so I don't think this patch
> does anything at all, unless I'm missing some context.
> 

Currently on upstream some MSR are intercepted: 
https://github.com/torvalds/linux/blob/master/arch/x86/kvm/vmx.c#L6838

In particular to this patch, the MSR_IA32_SPEC_CTRL intercept is 
disabled in 3/8: https://patchwork.kernel.org/patch/10151889/


> On Tue, Jan 30, 2018 at 5:21 AM, Mihai Carabas <mihai.carabas@oracle.com> wrote:
>> Hello Paolo,
>>
>> I've back ported this patch on 4.1, after adding the per-vcpu MSR bitmap.
>> Also enabled the SPEC_CTRL_MSR intercept if qemu instructed so [1].
>>
>> Reviewed-by: Mihai Carabas <mihai.carabas@oracle.com>
>>
>> [1]
>> +++ b/arch/x86/kvm/vmx.c
>> @@ -8391,6 +8391,16 @@ static struct kvm_vcpu *vmx_create_vcpu(struct kvm
>> *kvm, unsigned int id)
>>          vmx_disable_intercept_for_msr(msr_bitmap, MSR_IA32_PRED_CMD,
>> MSR_TYPE_R | MSR_TYPE_W);
>>          vmx_disable_intercept_for_msr(msr_bitmap, MSR_IA32_BNDCFGS,
>> MSR_TYPE_R | MSR_TYPE_W);
>>
>> +       /*
>> +        * If the physical CPU or the vCPU of this VM doesn't
>> +        * support SPEC_CTRL feature, catch each access to it.
>> +        */
>> +       if (!static_cpu_has(X86_FEATURE_SPEC_CTRL) ||
>> +            !guest_cpuid_has_spec_ctrl(&vmx->vcpu))
>> +               vmx_enable_intercept_for_msr(
>> +                       msr_bitmap,
>> +                       MSR_IA32_SPEC_CTRL,
>> +                       MSR_TYPE_R | MSR_TYPE_W);
>>
>>          /*
>>           * If PML is turned on, failure on enabling PML just results in
>> failure
>>
>>
>>
>> On 09.01.2018 14:03, Paolo Bonzini wrote:
>>>
>>> MSR_IA32_SPEC_CTRL is not available unless CPU[7,0].EDX[26] is 1.
>>> Check that against host CPUID or guest CPUID, respectively for
>>> host-initiated and guest-initiated accesses.
>>>
>>> Suggested-by: Jim Mattson <jmattson@google.com>
>>> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
>>> ---
>>>          This is for after X86_FEATURE_SPEC_CTRL is added to Linux, but
>>>          I still wanted to ack Jim's improvement.
>>>
>>>    arch/x86/kvm/svm.c | 8 ++++++++
>>>    arch/x86/kvm/vmx.c | 8 ++++++++
>>>    2 files changed, 16 insertions(+)
>>>
>>> diff --git a/arch/x86/kvm/svm.c b/arch/x86/kvm/svm.c
>>> index 97126c2bd663..3a646580d7c5 100644
>>> --- a/arch/x86/kvm/svm.c
>>> +++ b/arch/x86/kvm/svm.c
>>> @@ -3648,6 +3648,10 @@ static int svm_get_msr(struct kvm_vcpu *vcpu,
>>> struct msr_data *msr_info)
>>>                  msr_info->data = svm->nested.vm_cr_msr;
>>>                  break;
>>>          case MSR_IA32_SPEC_CTRL:
>>> +               if (!static_cpu_has(X86_FEATURE_SPEC_CTRL) ||
>>> +                   (!msr_info->host_initiated &&
>>> +                            !guest_cpuid_has(vcpu,
>>> X86_FEATURE_SPEC_CTRL)))
>>> +                       return 1;
>>>                  msr_info->data = svm->spec_ctrl;
>>>                  break;
>>>          case MSR_IA32_UCODE_REV:
>>> @@ -3806,6 +3810,10 @@ static int svm_set_msr(struct kvm_vcpu *vcpu,
>>> struct msr_data *msr)
>>>                  vcpu_unimpl(vcpu, "unimplemented wrmsr: 0x%x data
>>> 0x%llx\n", ecx, data);
>>>                  break;
>>>          case MSR_IA32_SPEC_CTRL:
>>> +               if (!static_cpu_has(X86_FEATURE_SPEC_CTRL) ||
>>> +                   (!msr_info->host_initiated &&
>>> +                            !guest_cpuid_has(vcpu,
>>> X86_FEATURE_SPEC_CTRL)))
>>> +                       return 1;
>>>                  svm->spec_ctrl = data;
>>>                  break;
>>>          case MSR_IA32_APICBASE:
>>> diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
>>> index 49b4a2d61603..42bc7ee293e4 100644
>>> --- a/arch/x86/kvm/vmx.c
>>> +++ b/arch/x86/kvm/vmx.c
>>> @@ -3368,6 +3368,10 @@ static int vmx_get_msr(struct kvm_vcpu *vcpu,
>>> struct msr_data *msr_info)
>>>                  msr_info->data = guest_read_tsc(vcpu);
>>>                  break;
>>>          case MSR_IA32_SPEC_CTRL:
>>> +               if (!static_cpu_has(X86_FEATURE_SPEC_CTRL) ||
>>> +                   (!msr_info->host_initiated &&
>>> +                    !guest_cpuid_has(vcpu, X86_FEATURE_SPEC_CTRL)))
>>> +                       return 1;
>>>                  msr_info->data = to_vmx(vcpu)->spec_ctrl;
>>>                  break;
>>>          case MSR_IA32_SYSENTER_CS:
>>> @@ -3510,6 +3514,10 @@ static int vmx_set_msr(struct kvm_vcpu *vcpu,
>>> struct msr_data *msr_info)
>>>                  kvm_write_tsc(vcpu, msr_info);
>>>                  break;
>>>          case MSR_IA32_SPEC_CTRL:
>>> +               if (!static_cpu_has(X86_FEATURE_SPEC_CTRL) ||
>>> +                   (!msr_info->host_initiated &&
>>> +                    !guest_cpuid_has(vcpu, X86_FEATURE_SPEC_CTRL)))
>>> +                       return 1;
>>>                  to_vmx(vcpu)->spec_ctrl = data;
>>>                  break;
>>>          case MSR_IA32_CR_PAT:
>>>
>>