Re: [PATCH] KVM: X86: #GP when guest attempts to write MCi banks w/o 0

[Jim Mattson]

Right. I was side-tracked by the code above yours for MCi_CTL.
However, does writing a non-zero value to MCi_STATUS/ADDR/MISC raise
#GP on AMD hardware? It is not clear from the APM. For MCi_MISC0, the
APM says:
"In some implementations, the MCi_MISC0 register is used for error
thresholding." Figure 9-8: Miscellaneous Information Register
(Thresholding Register Format) suggests that non-zero value can, in
fact, be written to MCi_MISC0 in such implementations.

Do any of the AMD contributors want to weigh in?

On Wed, Oct 18, 2017 at 6:39 PM, Wanpeng Li <wanpeng.li@xxxxxxxxxxx> wrote:
> Hi Jimï
> On 10/19/17 12:28 AM, Jim Mattson wrote:
>> The AMD APM says, "For each error-reporting register bank, software
>> should set the enable bits to 1 in the MCi_CTL register for the error
>> types it wants the processor to report. Software can write each
>> MCi_CTL with all 1s to enable all error-reporting mechanisms.' It does
>> not say that only all 1's or all 0's are allowed, and it implies that
>> any value is allowed.
>>
>> Since this is a vendor-agnostic function, the Intel-specific
>> constraints should only be applied when virtualizing Intel CPUs (in
>> particular, Intel P6 family CPUs). The same comment applies to the
>> existing constraints from commit 890ca9aefa78 ("KVM: Add MCE
>
> I have a discuss with the author of ("KVM: Add MCE support") face to
> face today in a kernel meeting held in our country. He told me his patch
> is against Intel architecture and not consider AMD when he introduced
> the patch. In addition, the difference which you mentioned is about
> MCi_CTL, however, my patches just focus on MCi_STATUS/ADDR/MISC.
>
> Regards,
> Wanpeng Li
>
>> support"), which were only partially relaxed by commit 114be429c8cd4
>> ("KVM: allow bit 10 to be cleared in MSR_IA32_MC4_CTL").
>>
>> On Wed, Oct 18, 2017 at 2:49 AM, Wanpeng Li <kernellwp@xxxxxxxxx> wrote:
>>> From: Wanpeng Li <wanpeng.li@xxxxxxxxxxx>
>>>
>>> SDM section 15.3.2.2~15.3.2.4 mentioned that MCi_STATUS/ADDR/MISC, when the
>>> registers are implemented, these registers can be cleared by explicitly writing
>>> 0s to these registers. Writing 1s to these registers will cause a
>>> general-protection exception.
>>>
>>> The mce is emulated in qemu, so just the guest attempts to write 1 to these
>>> registers should cause a #GP, this patch does it.
>>>
>>> Cc: Radim KrÄmÃÅ <rkrcmar@xxxxxxxxxx>
>>> Cc: Jim Mattson <jmattson@xxxxxxxxxx>
>>> Signed-off-by: Wanpeng Li <wanpeng.li@xxxxxxxxxxx>
>>> ---
>>>   arch/x86/kvm/x86.c | 9 +++++++--
>>>   1 file changed, 7 insertions(+), 2 deletions(-)
>>>
>>> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
>>> index 5669af0..a8680ea 100644
>>> --- a/arch/x86/kvm/x86.c
>>> +++ b/arch/x86/kvm/x86.c
>>> @@ -2006,10 +2006,12 @@ static void kvmclock_sync_fn(struct work_struct *work)
>>>                                          KVMCLOCK_SYNC_PERIOD);
>>>   }
>>>
>>> -static int set_msr_mce(struct kvm_vcpu *vcpu, u32 msr, u64 data)
>>> +static int set_msr_mce(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
>>>   {
>>>          u64 mcg_cap = vcpu->arch.mcg_cap;
>>>          unsigned bank_num = mcg_cap & 0xff;
>>> +       u32 msr = msr_info->index;
>>> +       u64 data = msr_info->data;
>>>
>>>          switch (msr) {
>>>          case MSR_IA32_MCG_STATUS:
>>> @@ -2034,6 +2036,9 @@ static int set_msr_mce(struct kvm_vcpu *vcpu, u32 msr, u64 data)
>>>                          if ((offset & 0x3) == 0 &&
>>>                              data != 0 && (data | (1 << 10)) != ~(u64)0)
>>>                                  return -1;
>>> +                       if (!msr_info->host_initiated &&
>>> +                               (offset & 0x3) != 0 && data != 0)
>>> +                               return -1;
>>>                          vcpu->arch.mce_banks[offset] = data;
>>>                          break;
>>>                  }
>>> @@ -2283,7 +2288,7 @@ int kvm_set_msr_common(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
>>>          case MSR_IA32_MCG_CTL:
>>>          case MSR_IA32_MCG_STATUS:
>>>          case MSR_IA32_MC0_CTL ... MSR_IA32_MCx_CTL(KVM_MAX_MCE_BANKS) - 1:
>>> -               return set_msr_mce(vcpu, msr, data);
>>> +               return set_msr_mce(vcpu, msr_info);
>>>
>>>          case MSR_K7_PERFCTR0 ... MSR_K7_PERFCTR3:
>>>          case MSR_P6_PERFCTR0 ... MSR_P6_PERFCTR1:
>>> --
>>> 2.7.4
>>>
>