Re: [PATCH] KVM/x86: pmu: Fix #GP condition check for RDPMC emulation

Paolo Bonzini
2020-07-08T15:31:23Z

On 08/07/20 17:18, Sean Christopherson wrote:
> On Wed, Jul 08, 2020 at 03:44:09PM +0800, Like Xu wrote:
>> in guest protected mode, if the current privilege level
>> is not 0 and the pce flag in the cr4 register is cleared,
>> we will inject a #gp for rdpmc usage.
>
> Wrapping at ~58 characters is a bit aggressive. checkpatch enforces 75
> chars, something near that would be prefereable.
>
>> Signed-off-by: Like Xu <like.xu@linux.intel.com>
>> ---
>> arch/x86/kvm/pmu.c | 5 +++++
>> 1 file changed, 5 insertions(+)
>>
>> diff --git a/arch/x86/kvm/pmu.c b/arch/x86/kvm/pmu.c
>> index b86346903f2e..d080d475c808 100644
>> --- a/arch/x86/kvm/pmu.c
>> +++ b/arch/x86/kvm/pmu.c
>> @@ -372,6 +372,11 @@ int kvm_pmu_rdpmc(struct kvm_vcpu *vcpu, unsigned idx, u64 *data)
>> if (!pmc)
>> return 1;
>>
>> + if ((kvm_x86_ops.get_cpl(vcpu) != 0) &&
>> + !(kvm_read_cr4(vcpu) & X86_CR4_PCE) &&
>> + (kvm_read_cr4(vcpu) & X86_CR0_PE))
>
> This reads CR4 but checks CR0.PE.
>
> And maybe put the X86_CR4_PCE check first so that it's the focus of the
> statement?

I'll squash this to fix it (I'm OOO next week and would like to get kvm/queue
sorted out these few days that I've left).

diff --git a/arch/x86/kvm/pmu.c b/arch/x86/kvm/pmu.c
index d080d475c808..67741d2a0308 100644
--- a/arch/x86/kvm/pmu.c
+++ b/arch/x86/kvm/pmu.c
@@ -372,9 +372,9 @@ int kvm_pmu_rdpmc(struct kvm_vcpu *vcpu, unsigned idx, u64 *data)
if (!pmc)
return 1;

- if ((kvm_x86_ops.get_cpl(vcpu) != 0) &&
- !(kvm_read_cr4(vcpu) & X86_CR4_PCE) &&
- (kvm_read_cr4(vcpu) & X86_CR0_PE))
+ if (!(kvm_read_cr4(vcpu) & X86_CR4_PCE) &&
+ (kvm_x86_ops.get_cpl(vcpu) != 0) &&
+ (kvm_read_cr0(vcpu) & X86_CR0_PE))
return 1;

*data = pmc_read_counter(pmc) & mask;

The order follows the SDM. I'm tempted to remove the CR0 check
altogether, since non-protected-mode always runs at CPL0 AFAIK, but let's
keep it close to what the manual says.

Paolo