linux-kernel - Re: [PATCH] KVM: SVM: Disable TDP MMU when running on Hyper-V

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Date:   Wed, 8 Mar 2023 01:00:32 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     Jeremi Piotrowski <jpiotrowski@...ux.microsoft.com>,
        linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Tianyu Lan <ltykernel@...il.com>,
        Michael Kelley <mikelley@...rosoft.com>
Subject: Re: [PATCH] KVM: SVM: Disable TDP MMU when running on Hyper-V

On Tue, Mar 7, 2023 at 6:36 PM Sean Christopherson <seanjc@...gle.com> wrote:
> Thinking about this more, I would rather revert commit 1e0c7d40758b ("KVM: SVM:
> hyper-v: Remote TLB flush for SVM") or fix the thing properly straitaway.  KVM
> doesn't magically handle the flushes correctly for the shadow/legacy MMU, KVM just
> happens to get lucky and not run afoul of the underlying bugs.

I don't think it's about luck---the legacy MMU's zapping/invalidation
seems to invoke the flush hypercall correctly:

Jeremi, did you ever track the call stack where
hyperv_nested_flush_guest_mapping is triggered?

Paolo

> The revert appears
> to be reasonably straightforward (see bottom).
>
> And _if_ we want to hack-a-fix it, then I would strongly prefer a very isolated,
> obviously hacky fix, e.g.
>
> diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
> index 36e4561554ca..a9ba4ae14fda 100644
> --- a/arch/x86/kvm/mmu/mmu.c
> +++ b/arch/x86/kvm/mmu/mmu.c
> @@ -5779,8 +5779,13 @@ void kvm_configure_mmu(bool enable_tdp, int tdp_forced_root_level,
>         tdp_root_level = tdp_forced_root_level;
>         max_tdp_level = tdp_max_root_level;
>
> +       /*
> +        * FIXME: Remove the enlightened TLB restriction when KVM properly
> +        * handles TLB flushes for said enlightenment.
> +        */.
>  #ifdef CONFIG_X86_64
> -       tdp_mmu_enabled = tdp_mmu_allowed && tdp_enabled;
> +       tdp_mmu_enabled = tdp_mmu_allowed && tdp_enabled &&
> +                         !(ms_hyperv.nested_features & HV_X64_NESTED_ENLIGHTENED_TLB);
>  #endif
>         /*
>          * max_huge_page_level reflects KVM's MMU capabilities irrespective
>
>
>
>
> The revert...
>
> ---
>  arch/x86/kvm/svm/svm.c          |  3 ---
>  arch/x86/kvm/svm/svm_onhyperv.h | 27 ---------------------------
>  2 files changed, 30 deletions(-)
>
> diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
> index 11068e8eb969..292650dc85a0 100644
> --- a/arch/x86/kvm/svm/svm.c
> +++ b/arch/x86/kvm/svm/svm.c
> @@ -1320,7 +1320,6 @@ static void init_vmcb(struct kvm_vcpu *vcpu)
>         if (sev_guest(vcpu->kvm))
>                 sev_init_vmcb(svm);
>
> -       svm_hv_init_vmcb(vmcb);
>         init_vmcb_after_set_cpuid(vcpu);
>
>         vmcb_mark_all_dirty(vmcb);
> @@ -4075,8 +4074,6 @@ static void svm_load_mmu_pgd(struct kvm_vcpu *vcpu, hpa_t root_hpa,
>                 svm->vmcb->control.nested_cr3 = __sme_set(root_hpa);
>                 vmcb_mark_dirty(svm->vmcb, VMCB_NPT);
>
> -               hv_track_root_tdp(vcpu, root_hpa);
> -
>                 cr3 = vcpu->arch.cr3;
>         } else if (root_level >= PT64_ROOT_4LEVEL) {
>                 cr3 = __sme_set(root_hpa) | kvm_get_active_pcid(vcpu);
> diff --git a/arch/x86/kvm/svm/svm_onhyperv.h b/arch/x86/kvm/svm/svm_onhyperv.h
> index 6981c1e9a809..5118fd273e73 100644
> --- a/arch/x86/kvm/svm/svm_onhyperv.h
> +++ b/arch/x86/kvm/svm/svm_onhyperv.h
> @@ -15,31 +15,8 @@ static struct kvm_x86_ops svm_x86_ops;
>
>  int svm_hv_enable_l2_tlb_flush(struct kvm_vcpu *vcpu);
>
> -static inline void svm_hv_init_vmcb(struct vmcb *vmcb)
> -{
> -       struct hv_vmcb_enlightenments *hve = &vmcb->control.hv_enlightenments;
> -
> -       BUILD_BUG_ON(sizeof(vmcb->control.hv_enlightenments) !=
> -                    sizeof(vmcb->control.reserved_sw));
> -
> -       if (npt_enabled &&
> -           ms_hyperv.nested_features & HV_X64_NESTED_ENLIGHTENED_TLB)
> -               hve->hv_enlightenments_control.enlightened_npt_tlb = 1;
> -
> -       if (ms_hyperv.nested_features & HV_X64_NESTED_MSR_BITMAP)
> -               hve->hv_enlightenments_control.msr_bitmap = 1;
> -}
> -
>  static inline void svm_hv_hardware_setup(void)
>  {
> -       if (npt_enabled &&
> -           ms_hyperv.nested_features & HV_X64_NESTED_ENLIGHTENED_TLB) {
> -               pr_info(KBUILD_MODNAME ": Hyper-V enlightened NPT TLB flush enabled\n");
> -               svm_x86_ops.tlb_remote_flush = hv_remote_flush_tlb;
> -               svm_x86_ops.tlb_remote_flush_with_range =
> -                               hv_remote_flush_tlb_with_range;
> -       }
> -
>         if (ms_hyperv.nested_features & HV_X64_NESTED_DIRECT_FLUSH) {
>                 int cpu;
>
> @@ -80,10 +57,6 @@ static inline void svm_hv_update_vp_id(struct vmcb *vmcb, struct kvm_vcpu *vcpu)
>  }
>  #else
>
> -static inline void svm_hv_init_vmcb(struct vmcb *vmcb)
> -{
> -}
> -
>  static inline void svm_hv_hardware_setup(void)
>  {
>  }
>
> base-commit: cb8748a781fe983e451f616ce4861a1c49ce79dd
> --
>