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

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Message-ID: <e285ccb3-29bd-dcb8-73d1-eeee11d72198@redhat.com>
Date:   Wed, 8 Jul 2020 17:31:14 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Sean Christopherson <sean.j.christopherson@...el.com>,
        Like Xu <like.xu@...ux.intel.com>
Cc:     kvm@...r.kernel.org, Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Joerg Roedel <joro@...tes.org>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] KVM/x86: pmu: Fix #GP condition check for RDPMC emulation

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@...ux.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