linux-kernel - Re: [PATCH v2 09/15] KVM: x86/tdp_mmu: Support mirror root for TDP MMU

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Date: Fri, 7 Jun 2024 20:27:12 +0000
From: "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>
To: "pbonzini@...hat.com" <pbonzini@...hat.com>
CC: "seanjc@...gle.com" <seanjc@...gle.com>, "Huang, Kai"
	<kai.huang@...el.com>, "sagis@...gle.com" <sagis@...gle.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>, "Aktas, Erdem"
	<erdemaktas@...gle.com>, "Zhao, Yan Y" <yan.y.zhao@...el.com>,
	"dmatlack@...gle.com" <dmatlack@...gle.com>, "kvm@...r.kernel.org"
	<kvm@...r.kernel.org>, "Yamahata, Isaku" <isaku.yamahata@...el.com>,
	"isaku.yamahata@...il.com" <isaku.yamahata@...il.com>
Subject: Re: [PATCH v2 09/15] KVM: x86/tdp_mmu: Support mirror root for TDP
 MMU

On Fri, 2024-06-07 at 10:46 +0200, Paolo Bonzini wrote:
> On Thu, May 30, 2024 at 11:07 PM Rick Edgecombe
> <rick.p.edgecombe@...el.com> wrote:
> > +static inline bool kvm_on_mirror(const struct kvm *kvm, enum kvm_process
> > process)
> > +{
> > +       if (!kvm_has_mirrored_tdp(kvm))
> > +               return false;
> > +
> > +       return process & KVM_PROCESS_PRIVATE;
> > +}
> > +
> > +static inline bool kvm_on_direct(const struct kvm *kvm, enum kvm_process
> > process)
> > +{
> > +       if (!kvm_has_mirrored_tdp(kvm))
> > +               return true;
> > +
> > +       return process & KVM_PROCESS_SHARED;
> > +}
> 
> These helpers don't add much, it's easier to just write
> kvm_process_to_root_types() as
> 
> if (process & KVM_PROCESS_SHARED)
>   ret |= KVM_DIRECT_ROOTS;
> if (process & KVM_PROCESS_PRIVATE)
>   ret |= kvm_has_mirror_tdp(kvm) ? KVM_MIRROR_ROOTS : KVM_DIRECT_ROOTS;
> 
> WARN_ON_ONCE(!ret);
> return ret;

The point of kvm_on_mirror() and kvm_on_direct() was to try to centralize TDX
specific checks in mmu.h. Whether that is a good idea aside, looking at it now
I'm not sure if it even does a good job.

I'll try it the way you suggest.

> 
> (Here I chose to rename kvm_has_mirrored_tdp to kvm_has_mirror_tdp;
> but if you prefer to call it kvm_has_external_tdp, it's just as
> readable. Whatever floats your boat).
> 
> >          struct kvm *kvm = vcpu->kvm;
> > -       struct kvm_mmu_page *root = root_to_sp(vcpu->arch.mmu->root.hpa);
> > +       enum kvm_tdp_mmu_root_types root_type =
> > tdp_mmu_get_fault_root_type(kvm, fault);
> > +       struct kvm_mmu_page *root = tdp_mmu_get_root(vcpu, root_type);
> 
> Please make this
> 
>   struct kvm_mmu_page *root = tdp_mmu_get_root_for_fault(vcpu, fault);

Yes, that seems clearer.


[snip]

> 
> > +static inline enum kvm_tdp_mmu_root_types
> > tdp_mmu_get_fault_root_type(struct kvm *kvm,
> > +                                                                     struct
> > kvm_page_fault *fault)
> > +{
> > +       if (fault->is_private)
> > +               return kvm_process_to_root_types(kvm, KVM_PROCESS_PRIVATE);
> > +       return KVM_DIRECT_ROOTS;
> > +}
> > +
> > +static inline struct kvm_mmu_page *tdp_mmu_get_root(struct kvm_vcpu *vcpu,
> > enum kvm_tdp_mmu_root_types type)
> > +{
> > +       if (type == KVM_MIRROR_ROOTS)
> > +               return root_to_sp(vcpu->arch.mmu->mirror_root_hpa);
> > +       return root_to_sp(vcpu->arch.mmu->root.hpa);
> > +}
> 
> static inline struct kvm_mmu_page *
> tdp_mmu_get_root_for_fault(struct kvm_vcpu *vcpu, struct kvm_page_fault
> *fault)
> {
>   hpa_t root_hpa;
>   if (unlikely(fault->is_private && kvm_has_mirror_tdp(kvm)))
>     root_hpa = vcpu->arch.mmu->mirror_root_hpa;
>   else
>     root_hpa = vcpu->arch.mmu->root.hpa;
>   return root_to_sp(root_hpa);
> }

I was not loving the amount of indirection here in the patch, but thought it
centralized the logic a bit better. This way seems good, given that the actual
logic is not that complex.