linux-kernel - Re: [PATCH v3 4/6] KVM: x86: Make use of kvm_read_cr*

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Message-ID: <7b428c85-5f20-a077-70b6-f361d2e7ffe7@grsecurity.net>
Date:   Mon, 20 Mar 2023 22:34:47 +0100
From:   Mathias Krause <minipli@...ecurity.net>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        Paolo Bonzini <pbonzini@...hat.com>
Subject: Re: [PATCH v3 4/6] KVM: x86: Make use of kvm_read_cr*_bits() when
 testing bits

On 15.03.23 23:18, Sean Christopherson wrote:
> On Wed, Feb 01, 2023, Mathias Krause wrote:
>> Make use of the kvm_read_cr{0,4}_bits() helper functions when we only
>> want to know the state of certain bits instead of the whole register.
>>
>> This not only makes the intend cleaner, it also avoids a VMREAD in case
>> the tested bits aren't guest owned.
>>
>> Signed-off-by: Mathias Krause <minipli@...ecurity.net>
>> ---
>>  arch/x86/kvm/pmu.c     | 4 ++--
>>  arch/x86/kvm/vmx/vmx.c | 4 ++--
>>  2 files changed, 4 insertions(+), 4 deletions(-)
>>
>> diff --git a/arch/x86/kvm/pmu.c b/arch/x86/kvm/pmu.c
>> index d939d3b84e6f..d9922277df67 100644
>> --- a/arch/x86/kvm/pmu.c
>> +++ b/arch/x86/kvm/pmu.c
>> @@ -439,9 +439,9 @@ int kvm_pmu_rdpmc(struct kvm_vcpu *vcpu, unsigned idx, u64 *data)
>>  	if (!pmc)
>>  		return 1;
>>  
>> -	if (!(kvm_read_cr4(vcpu) & X86_CR4_PCE) &&
>> +	if (!(kvm_read_cr4_bits(vcpu, X86_CR4_PCE)) &&
> 
> Purely as an FYI, I proposed adding helpers to query single CR0/CR4 bits in a
> separate thread[*].  No need to do anything on your end, I'll plan on applying
> this patch first and will handle whatever conflicts arise.
> 
> [*] https://lore.kernel.org/all/ZAuRec2NkC3+4jvD@google.com

Unfortunately, not all users of kvm_read_cr*_bits() only want to read a
single bit. There are a very few that read bit masks -- but you're
probably fully aware of this.

Thanks,
Mathias