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SubjectRe: [PATCH 04/22] kvm: mmu: Allocate and free TDP MMU roots
On Tue, Sep 29, 2020 at 11:06 PM Sean Christopherson
<sean.j.christopherson@intel.com> wrote:
>
> On Fri, Sep 25, 2020 at 02:22:44PM -0700, Ben Gardon wrote:
> static u64 __read_mostly shadow_nx_mask;
> > @@ -3597,10 +3592,14 @@ static void mmu_free_root_page(struct kvm *kvm, hpa_t *root_hpa,
> > if (!VALID_PAGE(*root_hpa))
> > return;
> >
> > - sp = to_shadow_page(*root_hpa & PT64_BASE_ADDR_MASK);
> > - --sp->root_count;
> > - if (!sp->root_count && sp->role.invalid)
> > - kvm_mmu_prepare_zap_page(kvm, sp, invalid_list);
> > + if (is_tdp_mmu_root(kvm, *root_hpa)) {
> > + kvm_tdp_mmu_put_root_hpa(kvm, *root_hpa);
> > + } else {
> > + sp = to_shadow_page(*root_hpa & PT64_BASE_ADDR_MASK);
> > + --sp->root_count;
> > + if (!sp->root_count && sp->role.invalid)
> > + kvm_mmu_prepare_zap_page(kvm, sp, invalid_list);
>
> Hmm, I see that future patches use put_tdp_mmu_root()/get_tdp_mmu_root(),
> but the code itself isn't specific to the TDP MMU. Even if this ends up
> being the only non-TDP user of get/put, I think it'd be worth making them
> common helpers, e.g.
>
> sp = to_shadow_page(*root_hpa & PT64_BASE_ADDR_MASK);
> if (mmu_put_root(sp) {
> if (is_tdp_mmu(...))
> kvm_tdp_mmu_free_root(kvm, sp);
> else if (sp->role.invalid)
> kvm_mmu_prepare_zap_page(kvm, sp, invalid_list);
> }
>
> > + }
> >
> > *root_hpa = INVALID_PAGE;
> > }
> > @@ -3691,7 +3690,13 @@ static int mmu_alloc_direct_roots(struct kvm_vcpu *vcpu)
> > unsigned i;
> >
> > if (shadow_root_level >= PT64_ROOT_4LEVEL) {
> > - root = mmu_alloc_root(vcpu, 0, 0, shadow_root_level, true);
> > + if (vcpu->kvm->arch.tdp_mmu_enabled) {
>
> I believe this will break 32-bit NPT. Or at a minimum, look weird. It'd
> be better to explicitly disable the TDP MMU on 32-bit KVM, then this becomes
>
> if (vcpu->kvm->arch.tdp_mmu_enabled) {
>
> } else if (shadow_root_level >= PT64_ROOT_4LEVEL) {
>
> } else {
>
> }
>

How does this break 32-bit NPT? I'm not sure I understand how we would
get into a bad state here because I'm not familiar with the specifics
of 32 bit NPT.

> > + root = kvm_tdp_mmu_get_vcpu_root_hpa(vcpu);
> > + } else {
> > + root = mmu_alloc_root(vcpu, 0, 0, shadow_root_level,
> > + true);
> > + }
>
> May not matter in the end, but the braces aren't needed.
>
> > +
> > if (!VALID_PAGE(root))
> > return -ENOSPC;
> > vcpu->arch.mmu->root_hpa = root;
> > diff --git a/arch/x86/kvm/mmu/mmu_internal.h b/arch/x86/kvm/mmu/mmu_internal.h
> > index 65bb110847858..530b7d893c7b3 100644
> > --- a/arch/x86/kvm/mmu/mmu_internal.h
> > +++ b/arch/x86/kvm/mmu/mmu_internal.h
> > @@ -41,8 +41,12 @@ struct kvm_mmu_page {
> >
> > /* Number of writes since the last time traversal visited this page. */
> > atomic_t write_flooding_count;
> > +
> > + bool tdp_mmu_page;
> > };
> >
> > +extern struct kmem_cache *mmu_page_header_cache;
> > +
> > static inline struct kvm_mmu_page *to_shadow_page(hpa_t shadow_page)
> > {
> > struct page *page = pfn_to_page(shadow_page >> PAGE_SHIFT);
> > @@ -69,6 +73,11 @@ bool kvm_mmu_slot_gfn_write_protect(struct kvm *kvm,
> > (((address) >> PT64_LEVEL_SHIFT(level)) & ((1 << PT64_LEVEL_BITS) - 1))
> > #define SHADOW_PT_INDEX(addr, level) PT64_INDEX(addr, level)
> >
> > +#define ACC_EXEC_MASK 1
> > +#define ACC_WRITE_MASK PT_WRITABLE_MASK
> > +#define ACC_USER_MASK PT_USER_MASK
> > +#define ACC_ALL (ACC_EXEC_MASK | ACC_WRITE_MASK | ACC_USER_MASK)
> > +
> > /* Functions for interpreting SPTEs */
> > kvm_pfn_t spte_to_pfn(u64 pte);
> > bool is_mmio_spte(u64 spte);
> > diff --git a/arch/x86/kvm/mmu/tdp_mmu.c b/arch/x86/kvm/mmu/tdp_mmu.c
> > index 8241e18c111e6..cdca829e42040 100644
> > --- a/arch/x86/kvm/mmu/tdp_mmu.c
> > +++ b/arch/x86/kvm/mmu/tdp_mmu.c
> > @@ -1,5 +1,7 @@
> > /* SPDX-License-Identifier: GPL-2.0 */
> >
> > +#include "mmu.h"
> > +#include "mmu_internal.h"
> > #include "tdp_mmu.h"
> >
> > static bool __read_mostly tdp_mmu_enabled = true;
> > @@ -25,10 +27,165 @@ void kvm_mmu_init_tdp_mmu(struct kvm *kvm)
> >
> > /* This should not be changed for the lifetime of the VM. */
> > kvm->arch.tdp_mmu_enabled = true;
> > +
> > + INIT_LIST_HEAD(&kvm->arch.tdp_mmu_roots);
> > }
> >
> > void kvm_mmu_uninit_tdp_mmu(struct kvm *kvm)
> > {
> > if (!kvm->arch.tdp_mmu_enabled)
> > return;
> > +
> > + WARN_ON(!list_empty(&kvm->arch.tdp_mmu_roots));
> > +}
> > +
> > +#define for_each_tdp_mmu_root(_kvm, _root) \
> > + list_for_each_entry(_root, &_kvm->arch.tdp_mmu_roots, link)
> > +
> > +bool is_tdp_mmu_root(struct kvm *kvm, hpa_t hpa)
> > +{
> > + struct kvm_mmu_page *root;
> > +
> > + if (!kvm->arch.tdp_mmu_enabled)
> > + return false;
> > +
> > + root = to_shadow_page(hpa);
> > +
> > + if (WARN_ON(!root))
> > + return false;
> > +
> > + return root->tdp_mmu_page;
>
> Why all the extra checks?
>
> > +}
> > +
> > +static void free_tdp_mmu_root(struct kvm *kvm, struct kvm_mmu_page *root)
> > +{
> > + lockdep_assert_held(&kvm->mmu_lock);
> > +
> > + WARN_ON(root->root_count);
> > + WARN_ON(!root->tdp_mmu_page);
> > +
> > + list_del(&root->link);
> > +
> > + free_page((unsigned long)root->spt);
> > + kmem_cache_free(mmu_page_header_cache, root);
> > +}
> > +
> > +static void put_tdp_mmu_root(struct kvm *kvm, struct kvm_mmu_page *root)
> > +{
> > + lockdep_assert_held(&kvm->mmu_lock);
> > +
> > + root->root_count--;
> > + if (!root->root_count)
> > + free_tdp_mmu_root(kvm, root);
> > +}
> > +
> > +static void get_tdp_mmu_root(struct kvm *kvm, struct kvm_mmu_page *root)
> > +{
> > + lockdep_assert_held(&kvm->mmu_lock);
> > + WARN_ON(!root->root_count);
> > +
> > + root->root_count++;
> > +}
> > +
> > +void kvm_tdp_mmu_put_root_hpa(struct kvm *kvm, hpa_t root_hpa)
> > +{
> > + struct kvm_mmu_page *root;
> > +
> > + root = to_shadow_page(root_hpa);
> > +
> > + if (WARN_ON(!root))
> > + return;
> > +
> > + put_tdp_mmu_root(kvm, root);
> > +}
> > +
> > +static struct kvm_mmu_page *find_tdp_mmu_root_with_role(
> > + struct kvm *kvm, union kvm_mmu_page_role role)
> > +{
> > + struct kvm_mmu_page *root;
> > +
> > + lockdep_assert_held(&kvm->mmu_lock);
> > + for_each_tdp_mmu_root(kvm, root) {
> > + WARN_ON(!root->root_count);
> > +
> > + if (root->role.word == role.word)
> > + return root;
> > + }
> > +
> > + return NULL;
> > +}
> > +
> > +static struct kvm_mmu_page *alloc_tdp_mmu_root(struct kvm_vcpu *vcpu,
> > + union kvm_mmu_page_role role)
> > +{
> > + struct kvm_mmu_page *new_root;
> > + struct kvm_mmu_page *root;
> > +
> > + new_root = kvm_mmu_memory_cache_alloc(
> > + &vcpu->arch.mmu_page_header_cache);
> > + new_root->spt = kvm_mmu_memory_cache_alloc(
> > + &vcpu->arch.mmu_shadow_page_cache);
> > + set_page_private(virt_to_page(new_root->spt), (unsigned long)new_root);
> > +
> > + new_root->role.word = role.word;
> > + new_root->root_count = 1;
> > + new_root->gfn = 0;
> > + new_root->tdp_mmu_page = true;
> > +
> > + spin_lock(&vcpu->kvm->mmu_lock);
> > +
> > + /* Check that no matching root exists before adding this one. */
> > + root = find_tdp_mmu_root_with_role(vcpu->kvm, role);
> > + if (root) {
> > + get_tdp_mmu_root(vcpu->kvm, root);
> > + spin_unlock(&vcpu->kvm->mmu_lock);
>
> Hrm, I'm not a big fan of dropping locks in the middle of functions, but the
> alternatives aren't great. :-/ Best I can come up with is
>
> if (root)
> get_tdp_mmu_root()
> else
> list_add();
>
> spin_unlock();
>
> if (root) {
> free_page()
> kmem_cache_free()
> } else {
> root = new_root;
> }
>
> return root;
>
> Not sure that's any better.
>
> > + free_page((unsigned long)new_root->spt);
> > + kmem_cache_free(mmu_page_header_cache, new_root);
> > + return root;
> > + }
> > +
> > + list_add(&new_root->link, &vcpu->kvm->arch.tdp_mmu_roots);
> > + spin_unlock(&vcpu->kvm->mmu_lock);
> > +
> > + return new_root;
> > +}
> > +
> > +static struct kvm_mmu_page *get_tdp_mmu_vcpu_root(struct kvm_vcpu *vcpu)
> > +{
> > + struct kvm_mmu_page *root;
> > + union kvm_mmu_page_role role;
> > +
> > + role = vcpu->arch.mmu->mmu_role.base;
> > + role.level = vcpu->arch.mmu->shadow_root_level;
> > + role.direct = true;
> > + role.gpte_is_8_bytes = true;
> > + role.access = ACC_ALL;
> > +
> > + spin_lock(&vcpu->kvm->mmu_lock);
> > +
> > + /* Search for an already allocated root with the same role. */
> > + root = find_tdp_mmu_root_with_role(vcpu->kvm, role);
> > + if (root) {
> > + get_tdp_mmu_root(vcpu->kvm, root);
> > + spin_unlock(&vcpu->kvm->mmu_lock);
>
> Rather than manually unlock and return, this can be
>
> if (root)
> get_tdp_mmju_root();
>
> spin_unlock()
>
> if (!root)
> root = alloc_tdp_mmu_root();
>
> return root;
>
> You could also add a helper to do the "get" along with the "find". Not sure
> if that's worth the code.
>
> > + return root;
> > + }
> > +
> > + spin_unlock(&vcpu->kvm->mmu_lock);
> > +
> > + /* If there is no appropriate root, allocate one. */
> > + root = alloc_tdp_mmu_root(vcpu, role);
> > +
> > + 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;
> > +
> > + return __pa(root->spt);
> > }
> > diff --git a/arch/x86/kvm/mmu/tdp_mmu.h b/arch/x86/kvm/mmu/tdp_mmu.h
> > index dd3764f5a9aa3..9274debffeaa1 100644
> > --- a/arch/x86/kvm/mmu/tdp_mmu.h
> > +++ b/arch/x86/kvm/mmu/tdp_mmu.h
> > @@ -7,4 +7,9 @@
> >
> > void kvm_mmu_init_tdp_mmu(struct kvm *kvm);
> > void kvm_mmu_uninit_tdp_mmu(struct kvm *kvm);
> > +
> > +bool is_tdp_mmu_root(struct kvm *kvm, hpa_t root);
> > +hpa_t kvm_tdp_mmu_get_vcpu_root_hpa(struct kvm_vcpu *vcpu);
> > +void kvm_tdp_mmu_put_root_hpa(struct kvm *kvm, hpa_t root_hpa);
> > +
> > #endif /* __KVM_X86_MMU_TDP_MMU_H */
> > --
> > 2.28.0.709.gb0816b6eb0-goog
> >

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