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

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Message-ID: <ZBnT8XhO9WuQFFK0@google.com>
Date:   Tue, 21 Mar 2023 08:57:37 -0700
From:   Sean Christopherson <seanjc@...gle.com>
To:     Mathias Krause <minipli@...ecurity.net>
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 Mon, Mar 20, 2023, Mathias Krause wrote:
> 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.

Yeah, we won't be able to get rid of kvm_read_cr*_bits() entirely, the goal is
purely to make code that does query a single bit slightly more readable/obvious.