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From:   David Matlack <dmatlack@google.com>
Date:   Fri, 15 Apr 2022 09:56:11 -0700
Message-ID: <CALzav=errSO8+EpeXE3F1CVbmttDqBigzJRGWc2UBU9=NLvtjw@mail.gmail.com>
Subject: Re: [PATCH] KVM: x86/mmu: Add RET_PF_CONTINUE to eliminate bool+int* "returns"
To:     Sean Christopherson <seanjc@google.com>
Cc:     Paolo Bonzini <pbonzini@redhat.com>,
        Vitaly Kuznetsov <vkuznets@redhat.com>,
        Wanpeng Li <wanpengli@tencent.com>,
        Jim Mattson <jmattson@google.com>,
        Joerg Roedel <joro@8bytes.org>, kvm list <kvm@vger.kernel.org>,
        LKML <linux-kernel@vger.kernel.org>,
        Chao Peng <chao.p.peng@linux.intel.com>
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On Thu, Apr 14, 2022 at 5:51 PM Sean Christopherson <seanjc@google.com> wrote:
>
> Add RET_PF_CONTINUE and use it in handle_abnormal_pfn() and
> kvm_faultin_pfn() to signal that the page fault handler should continue
> doing its thing.  Aside from being gross and inefficient, using a boolean
> return to signal continue vs. stop makes it extremely difficult to add
> more helpers and/or move existing code to a helper.
>
> E.g. hypothetically, if nested MMUs were to gain a separate page fault
> handler in the future, everything up to the "is self-modifying PTE" check
> can be shared by all shadow MMUs, but communicating up the stack whether
> to continue on or stop becomes a nightmare.
>
> More concretely, proposed support for private guest memory ran into a
> similar issue, where it'll be forced to forego a helper in order to yield
> sane code: https://lore.kernel.org/all/YkJbxiL%2FAz7olWlq@google.com.
>
> No functional change intended.
>
> Cc: David Matlack <dmatlack@google.com>
> Cc: Chao Peng <chao.p.peng@linux.intel.com>
> Signed-off-by: Sean Christopherson <seanjc@google.com>
> ---
>  arch/x86/kvm/mmu/mmu.c          | 51 ++++++++++++++-------------------
>  arch/x86/kvm/mmu/mmu_internal.h |  9 +++++-
>  arch/x86/kvm/mmu/paging_tmpl.h  |  6 ++--
>  3 files changed, 34 insertions(+), 32 deletions(-)
>
> diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
> index 69a30d6d1e2b..cb2982c6b513 100644
> --- a/arch/x86/kvm/mmu/mmu.c
> +++ b/arch/x86/kvm/mmu/mmu.c
> @@ -2972,14 +2972,12 @@ static int kvm_handle_bad_page(struct kvm_vcpu *vcpu, gfn_t gfn, kvm_pfn_t pfn)
>         return -EFAULT;
>  }
>
> -static bool handle_abnormal_pfn(struct kvm_vcpu *vcpu, struct kvm_page_fault *fault,
> -                               unsigned int access, int *ret_val)
> +static int handle_abnormal_pfn(struct kvm_vcpu *vcpu, struct kvm_page_fault *fault,
> +                              unsigned int access)
>  {
>         /* The pfn is invalid, report the error! */
> -       if (unlikely(is_error_pfn(fault->pfn))) {
> -               *ret_val = kvm_handle_bad_page(vcpu, fault->gfn, fault->pfn);
> -               return true;
> -       }
> +       if (unlikely(is_error_pfn(fault->pfn)))
> +               return kvm_handle_bad_page(vcpu, fault->gfn, fault->pfn);
>
>         if (unlikely(!fault->slot)) {
>                 gva_t gva = fault->is_tdp ? 0 : fault->addr;
> @@ -2991,13 +2989,11 @@ static bool handle_abnormal_pfn(struct kvm_vcpu *vcpu, struct kvm_page_fault *fa
>                  * touching the shadow page tables as attempting to install an
>                  * MMIO SPTE will just be an expensive nop.
>                  */
> -               if (unlikely(!shadow_mmio_value)) {
> -                       *ret_val = RET_PF_EMULATE;
> -                       return true;
> -               }
> +               if (unlikely(!shadow_mmio_value))
> +                       return RET_PF_EMULATE;
>         }
>
> -       return false;
> +       return RET_PF_CONTINUE;
>  }
>
>  static bool page_fault_can_be_fast(struct kvm_page_fault *fault)
> @@ -3888,7 +3884,7 @@ static bool kvm_arch_setup_async_pf(struct kvm_vcpu *vcpu, gpa_t cr2_or_gpa,
>                                   kvm_vcpu_gfn_to_hva(vcpu, gfn), &arch);
>  }
>
> -static bool kvm_faultin_pfn(struct kvm_vcpu *vcpu, struct kvm_page_fault *fault, int *r)
> +static int kvm_faultin_pfn(struct kvm_vcpu *vcpu, struct kvm_page_fault *fault)
>  {
>         struct kvm_memory_slot *slot = fault->slot;
>         bool async;
> @@ -3899,7 +3895,7 @@ static bool kvm_faultin_pfn(struct kvm_vcpu *vcpu, struct kvm_page_fault *fault,
>          * be zapped before KVM inserts a new MMIO SPTE for the gfn.
>          */
>         if (slot && (slot->flags & KVM_MEMSLOT_INVALID))
> -               goto out_retry;
> +               return RET_PF_RETRY;
>
>         if (!kvm_is_visible_memslot(slot)) {
>                 /* Don't expose private memslots to L2. */
> @@ -3907,7 +3903,7 @@ static bool kvm_faultin_pfn(struct kvm_vcpu *vcpu, struct kvm_page_fault *fault,
>                         fault->slot = NULL;
>                         fault->pfn = KVM_PFN_NOSLOT;
>                         fault->map_writable = false;
> -                       return false;
> +                       return RET_PF_CONTINUE;
>                 }
>                 /*
>                  * If the APIC access page exists but is disabled, go directly
> @@ -3916,10 +3912,8 @@ static bool kvm_faultin_pfn(struct kvm_vcpu *vcpu, struct kvm_page_fault *fault,
>                  * when the AVIC is re-enabled.
>                  */
>                 if (slot && slot->id == APIC_ACCESS_PAGE_PRIVATE_MEMSLOT &&
> -                   !kvm_apicv_activated(vcpu->kvm)) {
> -                       *r = RET_PF_EMULATE;
> -                       return true;
> -               }
> +                   !kvm_apicv_activated(vcpu->kvm))
> +                       return RET_PF_EMULATE;
>         }
>
>         async = false;
> @@ -3927,26 +3921,23 @@ static bool kvm_faultin_pfn(struct kvm_vcpu *vcpu, struct kvm_page_fault *fault,
>                                           fault->write, &fault->map_writable,
>                                           &fault->hva);
>         if (!async)
> -               return false; /* *pfn has correct page already */
> +               return RET_PF_CONTINUE; /* *pfn has correct page already */
>
>         if (!fault->prefetch && kvm_can_do_async_pf(vcpu)) {
>                 trace_kvm_try_async_get_page(fault->addr, fault->gfn);
>                 if (kvm_find_async_pf_gfn(vcpu, fault->gfn)) {
>                         trace_kvm_async_pf_doublefault(fault->addr, fault->gfn);
>                         kvm_make_request(KVM_REQ_APF_HALT, vcpu);
> -                       goto out_retry;
> -               } else if (kvm_arch_setup_async_pf(vcpu, fault->addr, fault->gfn))
> -                       goto out_retry;
> +                       return RET_PF_RETRY;
> +               } else if (kvm_arch_setup_async_pf(vcpu, fault->addr, fault->gfn)) {
> +                       return RET_PF_RETRY;
> +               }
>         }
>
>         fault->pfn = __gfn_to_pfn_memslot(slot, fault->gfn, false, NULL,
>                                           fault->write, &fault->map_writable,
>                                           &fault->hva);
> -       return false;
> -
> -out_retry:
> -       *r = RET_PF_RETRY;
> -       return true;
> +       return RET_PF_CONTINUE;
>  }
>
>  /*
> @@ -4001,10 +3992,12 @@ static int direct_page_fault(struct kvm_vcpu *vcpu, struct kvm_page_fault *fault
>         mmu_seq = vcpu->kvm->mmu_notifier_seq;
>         smp_rmb();
>
> -       if (kvm_faultin_pfn(vcpu, fault, &r))
> +       r = kvm_faultin_pfn(vcpu, fault);
> +       if (r != RET_PF_CONTINUE)
>                 return r;
>
> -       if (handle_abnormal_pfn(vcpu, fault, ACC_ALL, &r))
> +       r = handle_abnormal_pfn(vcpu, fault, ACC_ALL);
> +       if (r != RET_PF_CONTINUE)
>                 return r;
>
>         r = RET_PF_RETRY;
> diff --git a/arch/x86/kvm/mmu/mmu_internal.h b/arch/x86/kvm/mmu/mmu_internal.h
> index 1bff453f7cbe..c0e502b17ef7 100644
> --- a/arch/x86/kvm/mmu/mmu_internal.h
> +++ b/arch/x86/kvm/mmu/mmu_internal.h
> @@ -143,6 +143,7 @@ unsigned int pte_list_count(struct kvm_rmap_head *rmap_head);
>  /*
>   * Return values of handle_mmio_page_fault, mmu.page_fault, and fast_page_fault().
>   *
> + * RET_PF_CONTINUE: So far, so good, keep handling the page fault.
>   * RET_PF_RETRY: let CPU fault again on the address.
>   * RET_PF_EMULATE: mmio page fault, emulate the instruction directly.
>   * RET_PF_INVALID: the spte is invalid, let the real page fault path update it.

RET_PF_CONTINUE and RET_PF_INVALID are pretty similar, they both
indicate to the PF handler that it should keep going. What do you
think about taking this a step further and removing RET_PF_INVALID and
RET_PF_CONTINUE and using 0 instead? That way we can replace:

  if (ret != RET_PF_CONTINUE)
          return r;

and

  if (ret != RET_PF_INVALID)
          return r;

with

  if (r)
          return r;

?

> @@ -151,9 +152,15 @@ unsigned int pte_list_count(struct kvm_rmap_head *rmap_head);
>   *
>   * Any names added to this enum should be exported to userspace for use in
>   * tracepoints via TRACE_DEFINE_ENUM() in mmutrace.h

Please add RET_PF_CONTINUE to mmutrace.h, e.g.

  TRACE_DEFINE_ENUM(RET_PF_CONTINUE);

It doesn't matter in practice (yet) since the trace enums are only
used for trace_fast_page_fault, which does not return RET_PF_CONTINUE.
But it would be good to keep it up to date.

> + *
> + * Note, all values must be greater than or equal to zero so as not to encroach
> + * on -errno return values.  Somewhat arbitrarily use '0' for CONTINUE, which
> + * will allow for efficient machine code when checking for CONTINUE, e.g.
> + * "TEST %rax, %rax, JNZ", as all "stop!" values are non-zero.
>   */
>  enum {
> -       RET_PF_RETRY = 0,
> +       RET_PF_CONTINUE = 0,
> +       RET_PF_RETRY,
>         RET_PF_EMULATE,
>         RET_PF_INVALID,
>         RET_PF_FIXED,
> diff --git a/arch/x86/kvm/mmu/paging_tmpl.h b/arch/x86/kvm/mmu/paging_tmpl.h
> index 66f1acf153c4..7038273d04ab 100644
> --- a/arch/x86/kvm/mmu/paging_tmpl.h
> +++ b/arch/x86/kvm/mmu/paging_tmpl.h
> @@ -838,10 +838,12 @@ static int FNAME(page_fault)(struct kvm_vcpu *vcpu, struct kvm_page_fault *fault
>         mmu_seq = vcpu->kvm->mmu_notifier_seq;
>         smp_rmb();
>
> -       if (kvm_faultin_pfn(vcpu, fault, &r))
> +       r = kvm_faultin_pfn(vcpu, fault);
> +       if (r != RET_PF_CONTINUE)
>                 return r;
>
> -       if (handle_abnormal_pfn(vcpu, fault, walker.pte_access, &r))
> +       r = handle_abnormal_pfn(vcpu, fault, walker.pte_access);
> +       if (r != RET_PF_CONTINUE)
>                 return r;
>
>         /*
>
> base-commit: 150866cd0ec871c765181d145aa0912628289c8a
> --
> 2.36.0.rc0.470.gd361397f0d-goog
>