linux-kernel - Re: [PATCH v2 01/25] KVM: x86/mmu: avoid indirect call for get

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Date:   Tue, 8 Mar 2022 16:53:10 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        dmatlack@...gle.com
Subject: Re: [PATCH v2 01/25] KVM: x86/mmu: avoid indirect call for get_cr3

On Tue, Mar 08, 2022, Paolo Bonzini wrote:
> On 3/8/22 17:32, Sean Christopherson wrote:
> > 
> > Aha!  An idea that would provide line of sight to avoiding retpoline in all cases
> > once we use static_call() for nested_ops, which I really want to do...  Drop the
> > mmu hook entirely and replace it with:
> > 
> > static inline kvm_mmu_get_guest_pgd(struct kvm_vcpu *vcpu)
> > {
> > 	if (!mmu_is_nested(vcpu))
> > 		return kvm_read_cr3(vcpu);
> > 	else
> > 		return kvm_x86_ops.nested_ops->get_guest_pgd(vcpu);
> > }
> 
> Makes sense, but I think you mean
> 
> static inline unsigned long kvm_mmu_get_guest_pgd(struct kvm_vcpu *vcpu,
> 						  struct kvm_mmu *mmu)
> {
> 	if (unlikely(vcpu == &vcpu->arch.guest_mmu))

Well, not that certainly :-)

	if (mmu == &vcpu->arch.guest_mmu)

But you're right, we need to be able to do kvm_read_cr3() for the actual nested_mmu.

> 		return kvm_x86_ops.nested_ops->get_guest_pgd(vcpu);
> 	else
> 		return kvm_read_cr3(vcpu);
> }
> 
> ?