linux-kernel - Re: [PATCH v2 14/18] KVM: x86/mmu: avoid indirect call for get

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Message-ID: <YhAB1d1/nQbx6yvk@google.com>
Date:   Fri, 18 Feb 2022 20:30:13 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org
Subject: Re: [PATCH v2 14/18] KVM: x86/mmu: avoid indirect call for get_cr3

On Thu, Feb 17, 2022, Paolo Bonzini wrote:
> Most of the time, calls to get_guest_pgd result in calling
> kvm_read_cr3 (the exception is only nested TDP).  Hardcode
> the default instead of using the get_cr3 function, avoiding
> a retpoline if they are enabled.
> 
> Signed-off-by: Paolo Bonzini <pbonzini@...hat.com>
> ---
>  arch/x86/kvm/mmu.h             | 13 +++++++++++++
>  arch/x86/kvm/mmu/mmu.c         | 15 +++++----------
>  arch/x86/kvm/mmu/paging_tmpl.h |  2 +-
>  arch/x86/kvm/x86.c             |  2 +-
>  4 files changed, 20 insertions(+), 12 deletions(-)
> 
> diff --git a/arch/x86/kvm/mmu.h b/arch/x86/kvm/mmu.h
> index 1d0c1904d69a..1808d6814ddb 100644
> --- a/arch/x86/kvm/mmu.h
> +++ b/arch/x86/kvm/mmu.h
> @@ -116,6 +116,19 @@ static inline void kvm_mmu_load_pgd(struct kvm_vcpu *vcpu)
>  					  vcpu->arch.mmu->shadow_root_level);
>  }
>  
> +static inline gpa_t __kvm_mmu_get_guest_pgd(struct kvm_vcpu *vcpu, struct kvm_mmu *mmu)
> +{

I'd prefer to do what we do for page faults.  That approach avoids the need for a
comment to document NULL and avoids a conditional when RETPOLINE is not enabled.

Might be worth renaming get_cr3 => get_guest_cr3 though.

#ifdef CONFIG_RETPOLINE
	if (mmu->get_guest_pgd = get_guest_cr3)
		return kvm_read_cr3(vcpu);
#endif
	return mmu->get_guest_pgd(vcpu);


> +	if (!mmu->get_guest_pgd)
> +		return kvm_read_cr3(vcpu);
> +	else
> +		return mmu->get_guest_pgd(vcpu);
> +}
> +
> +static inline gpa_t kvm_mmu_get_guest_pgd(struct kvm_vcpu *vcpu)
> +{
> +	return __kvm_mmu_get_guest_pgd(vcpu, vcpu->arch.mmu);

I'd much prefer we don't provide an @vcpu-only variant and force the caller to
provide the mmu.