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From: Ben Gardon <bgardon@google.com>
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 <kvm@vger.kernel.org>, LKML <linux-kernel@vger.kernel.org>,
	Brijesh Singh <brijesh.singh@amd.com>,
	Tom Lendacky <thomas.lendacky@amd.com>
Subject: Re: [PATCH 03/15] KVM: x86/mmu: Ensure MMU pages are available when allocating roots
Date: Tue, 2 Mar 2021 16:21:46 -0800	[thread overview]
Message-ID: <CANgfPd95G-01ObzeKeMTUu0yXBJ=0+ZGQwr_5WCNH-NmR03f9w@mail.gmail.com> (raw)
In-Reply-To: <20210302184540.2829328-4-seanjc@google.com>

On Tue, Mar 2, 2021 at 10:46 AM Sean Christopherson <seanjc@google.com> wrote:
>
> Hold the mmu_lock for write for the entire duration of allocating and
> initializing an MMU's roots.  This ensures there are MMU pages available
> and thus prevents root allocations from failing.  That in turn fixes a
> bug where KVM would fail to free valid PAE roots if a one of the later
> roots failed to allocate.
>
> Note, KVM still leaks the PAE roots if the lm_root allocation fails.
> This will be addressed in a future commit.
>
> Cc: Ben Gardon <bgardon@google.com>
> Signed-off-by: Sean Christopherson <seanjc@google.com>

Reviewed-by: Ben Gardon <bgardon@google.com>

Very tidy cleanup!

> ---
>  arch/x86/kvm/mmu/mmu.c     | 41 ++++++++++++--------------------------
>  arch/x86/kvm/mmu/tdp_mmu.c | 23 +++++----------------
>  2 files changed, 18 insertions(+), 46 deletions(-)
>
> diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
> index 2ed3fac1244e..1f129001a30c 100644
> --- a/arch/x86/kvm/mmu/mmu.c
> +++ b/arch/x86/kvm/mmu/mmu.c
> @@ -2398,6 +2398,9 @@ static int make_mmu_pages_available(struct kvm_vcpu *vcpu)
>  {
>         unsigned long avail = kvm_mmu_available_pages(vcpu->kvm);
>
> +       /* Ensure all four PAE roots can be allocated in a single pass. */
> +       BUILD_BUG_ON(KVM_MIN_FREE_MMU_PAGES < 4);
> +

For a second I thought that this should be 5 since a page is needed to
hold the 4 PAE roots, but that page is allocated at vCPU creation and
reused, so no need to check for it here.

>         if (likely(avail >= KVM_MIN_FREE_MMU_PAGES))
>                 return 0;
>
> @@ -3220,16 +3223,9 @@ static hpa_t mmu_alloc_root(struct kvm_vcpu *vcpu, gfn_t gfn, gva_t gva,
>  {
>         struct kvm_mmu_page *sp;
>
> -       write_lock(&vcpu->kvm->mmu_lock);
> -
> -       if (make_mmu_pages_available(vcpu)) {
> -               write_unlock(&vcpu->kvm->mmu_lock);
> -               return INVALID_PAGE;
> -       }
>         sp = kvm_mmu_get_page(vcpu, gfn, gva, level, direct, ACC_ALL);
>         ++sp->root_count;
>
> -       write_unlock(&vcpu->kvm->mmu_lock);
>         return __pa(sp->spt);
>  }
>
> @@ -3241,16 +3237,10 @@ static int mmu_alloc_direct_roots(struct kvm_vcpu *vcpu)
>
>         if (is_tdp_mmu_enabled(vcpu->kvm)) {
>                 root = kvm_tdp_mmu_get_vcpu_root_hpa(vcpu);
> -
> -               if (!VALID_PAGE(root))
> -                       return -ENOSPC;
>                 vcpu->arch.mmu->root_hpa = root;
>         } else if (shadow_root_level >= PT64_ROOT_4LEVEL) {
>                 root = mmu_alloc_root(vcpu, 0, 0, shadow_root_level,
>                                       true);
> -
> -               if (!VALID_PAGE(root))
> -                       return -ENOSPC;

There's so much going on in mmu_alloc_root that removing this check
makes me nervous, but I think it should be safe.
I checked though the function because I was worried it might yield
somewhere in there, which could result in the page cache being emptied
and the allocation failing, but I don't think mmu_alloc_root this
function will yield.

>                 vcpu->arch.mmu->root_hpa = root;
>         } else if (shadow_root_level == PT32E_ROOT_LEVEL) {
>                 for (i = 0; i < 4; ++i) {
> @@ -3258,8 +3248,6 @@ static int mmu_alloc_direct_roots(struct kvm_vcpu *vcpu)
>
>                         root = mmu_alloc_root(vcpu, i << (30 - PAGE_SHIFT),
>                                               i << 30, PT32_ROOT_LEVEL, true);
> -                       if (!VALID_PAGE(root))
> -                               return -ENOSPC;
>                         vcpu->arch.mmu->pae_root[i] = root | PT_PRESENT_MASK;
>                 }
>                 vcpu->arch.mmu->root_hpa = __pa(vcpu->arch.mmu->pae_root);
> @@ -3294,8 +3282,6 @@ static int mmu_alloc_shadow_roots(struct kvm_vcpu *vcpu)
>
>                 root = mmu_alloc_root(vcpu, root_gfn, 0,
>                                       vcpu->arch.mmu->shadow_root_level, false);
> -               if (!VALID_PAGE(root))
> -                       return -ENOSPC;
>                 vcpu->arch.mmu->root_hpa = root;
>                 goto set_root_pgd;
>         }
> @@ -3325,6 +3311,7 @@ static int mmu_alloc_shadow_roots(struct kvm_vcpu *vcpu)
>
>         for (i = 0; i < 4; ++i) {
>                 MMU_WARN_ON(VALID_PAGE(vcpu->arch.mmu->pae_root[i]));
> +
>                 if (vcpu->arch.mmu->root_level == PT32E_ROOT_LEVEL) {
>                         pdptr = vcpu->arch.mmu->get_pdptr(vcpu, i);
>                         if (!(pdptr & PT_PRESENT_MASK)) {
> @@ -3338,8 +3325,6 @@ static int mmu_alloc_shadow_roots(struct kvm_vcpu *vcpu)
>
>                 root = mmu_alloc_root(vcpu, root_gfn, i << 30,
>                                       PT32_ROOT_LEVEL, false);
> -               if (!VALID_PAGE(root))
> -                       return -ENOSPC;
>                 vcpu->arch.mmu->pae_root[i] = root | pm_mask;
>         }
>         vcpu->arch.mmu->root_hpa = __pa(vcpu->arch.mmu->pae_root);
> @@ -3373,14 +3358,6 @@ static int mmu_alloc_shadow_roots(struct kvm_vcpu *vcpu)
>         return 0;
>  }
>
> -static int mmu_alloc_roots(struct kvm_vcpu *vcpu)
> -{
> -       if (vcpu->arch.mmu->direct_map)
> -               return mmu_alloc_direct_roots(vcpu);
> -       else
> -               return mmu_alloc_shadow_roots(vcpu);
> -}
> -
>  void kvm_mmu_sync_roots(struct kvm_vcpu *vcpu)
>  {
>         int i;
> @@ -4822,7 +4799,15 @@ int kvm_mmu_load(struct kvm_vcpu *vcpu)
>         r = mmu_topup_memory_caches(vcpu, !vcpu->arch.mmu->direct_map);
>         if (r)
>                 goto out;
> -       r = mmu_alloc_roots(vcpu);
> +       write_lock(&vcpu->kvm->mmu_lock);
> +       if (make_mmu_pages_available(vcpu))
> +               r = -ENOSPC;
> +       else if (vcpu->arch.mmu->direct_map)
> +               r = mmu_alloc_direct_roots(vcpu);
> +       else
> +               r = mmu_alloc_shadow_roots(vcpu);
> +       write_unlock(&vcpu->kvm->mmu_lock);
> +
>         kvm_mmu_sync_roots(vcpu);
>         if (r)
>                 goto out;
> diff --git a/arch/x86/kvm/mmu/tdp_mmu.c b/arch/x86/kvm/mmu/tdp_mmu.c
> index 70226e0875fe..50ef757c5586 100644
> --- a/arch/x86/kvm/mmu/tdp_mmu.c
> +++ b/arch/x86/kvm/mmu/tdp_mmu.c
> @@ -137,22 +137,21 @@ static struct kvm_mmu_page *alloc_tdp_mmu_page(struct kvm_vcpu *vcpu, gfn_t gfn,
>         return sp;
>  }
>
> -static struct kvm_mmu_page *get_tdp_mmu_vcpu_root(struct kvm_vcpu *vcpu)
> +hpa_t kvm_tdp_mmu_get_vcpu_root_hpa(struct kvm_vcpu *vcpu)
>  {
>         union kvm_mmu_page_role role;
>         struct kvm *kvm = vcpu->kvm;
>         struct kvm_mmu_page *root;
>
> +       lockdep_assert_held_write(&kvm->mmu_lock);
> +
>         role = page_role_for_level(vcpu, vcpu->arch.mmu->shadow_root_level);
>
> -       write_lock(&kvm->mmu_lock);
> -
>         /* Check for an existing root before allocating a new one. */
>         for_each_tdp_mmu_root(kvm, root) {
>                 if (root->role.word == role.word) {
>                         kvm_mmu_get_root(kvm, root);
> -                       write_unlock(&kvm->mmu_lock);
> -                       return root;
> +                       goto out;
>                 }
>         }
>
> @@ -161,19 +160,7 @@ static struct kvm_mmu_page *get_tdp_mmu_vcpu_root(struct kvm_vcpu *vcpu)
>
>         list_add(&root->link, &kvm->arch.tdp_mmu_roots);
>
> -       write_unlock(&kvm->mmu_lock);
> -
> -       return root;
> -}
> -
> -hpa_t kvm_tdp_mmu_get_vcpu_root_hpa(struct kvm_vcpu *vcpu)
> -{
> -       struct kvm_mmu_page *root;
> -
> -       root = get_tdp_mmu_vcpu_root(vcpu);
> -       if (!root)
> -               return INVALID_PAGE;
> -
> +out:
>         return __pa(root->spt);
>  }
>
> --
> 2.30.1.766.gb4fecdf3b7-goog
>

  reply	other threads:[~2021-03-03 11:03 UTC|newest]

Thread overview: 19+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-03-02 18:45 [PATCH 00/15] KVM: x86/mmu: Lots of bug fixes Sean Christopherson
2021-03-02 18:45 ` [PATCH 01/15] KVM: nSVM: Set the shadow root level to the TDP level for nested NPT Sean Christopherson
2021-03-02 18:45 ` [PATCH 02/15] KVM: x86/mmu: Alloc page for PDPTEs when shadowing 32-bit NPT with 64-bit Sean Christopherson
2021-03-03 17:28   ` Ben Gardon
2021-03-02 18:45 ` [PATCH 03/15] KVM: x86/mmu: Ensure MMU pages are available when allocating roots Sean Christopherson
2021-03-03  0:21   ` Ben Gardon [this message]
2021-03-03 16:46     ` Sean Christopherson
2021-03-02 18:45 ` [PATCH 04/15] KVM: x86/mmu: Allocate the lm_root before allocating PAE roots Sean Christopherson
2021-03-02 18:45 ` [PATCH 05/15] KVM: x86/mmu: Check PDPTRs " Sean Christopherson
2021-03-02 18:45 ` [PATCH 06/15] KVM: x86/mmu: Fix and unconditionally enable WARNs to detect PAE leaks Sean Christopherson
2021-03-02 18:45 ` [PATCH 07/15] KVM: x86/mmu: Use '0' as the one and only value for an invalid PAE root Sean Christopherson
2021-03-02 18:45 ` [PATCH 08/15] KVM: x86/mmu: Set the C-bit in the PDPTRs and LM pseudo-PDPTRs Sean Christopherson
2021-03-02 18:45 ` [PATCH 09/15] KVM: x86/mmu: Mark the PAE roots as decrypted for shadow paging Sean Christopherson
2021-03-02 18:45 ` [PATCH 10/15] KVM: SVM: Don't strip the C-bit from CR2 on #PF interception Sean Christopherson
2021-03-02 18:45 ` [PATCH 11/15] KVM: nVMX: Defer the MMU reload to the normal path on an EPTP switch Sean Christopherson
2021-03-02 18:45 ` [PATCH 12/15] KVM: x86: Defer the MMU unload to the normal path on an global INVPCID Sean Christopherson
2021-03-02 18:45 ` [PATCH 13/15] KVM: x86/mmu: Unexport MMU load/unload functions Sean Christopherson
2021-03-02 18:45 ` [PATCH 14/15] KVM: x86/mmu: Sync roots after MMU load iff load as successful Sean Christopherson
2021-03-02 18:45 ` [PATCH 15/15] KVM: x86/mmu: WARN on NULL pae_root and bad shadow root level Sean Christopherson

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