Re: [PATCH 03/15] KVM: x86/mmu: Ensure MMU pages are available when allocating roots

From: Ben Gardon
Date: Wed Mar 03 2021 - 06:02:27 EST

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On Tue, Mar 2, 2021 at 10:46 AM Sean Christopherson <seanjc@xxxxxxxxxx> 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@xxxxxxxxxx>
> Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>

Reviewed-by: Ben Gardon <bgardon@xxxxxxxxxx>

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
>

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Previous message: Pavel Tatashin: "[PATCH v12 02/17] arm64: hyp-stub: Move invalid vector entries into the vectors"
In reply to: Sean Christopherson: "[PATCH 03/15] KVM: x86/mmu: Ensure MMU pages are available when allocating roots"
Next in thread: Sean Christopherson: "Re: [PATCH 03/15] KVM: x86/mmu: Ensure MMU pages are available when allocating roots"
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