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Date: Fri, 4 Mar 2022 01:16:59 +0000
From: Mingwei Zhang <mizhang@...gle.com>
To: Paolo Bonzini <pbonzini@...hat.com>
Cc: linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
Sean Christopherson <seanjc@...gle.com>,
Vitaly Kuznetsov <vkuznets@...hat.com>,
Wanpeng Li <wanpengli@...cent.com>,
Jim Mattson <jmattson@...gle.com>,
Joerg Roedel <joro@...tes.org>,
David Hildenbrand <david@...hat.com>,
David Matlack <dmatlack@...gle.com>,
Ben Gardon <bgardon@...gle.com>
Subject: Re: [PATCH v4 18/30] KVM: x86/mmu: Zap only TDP MMU leafs in
kvm_zap_gfn_range()
On Thu, Mar 03, 2022, Paolo Bonzini wrote:
> From: Sean Christopherson <seanjc@...gle.com>
>
> Zap only leaf SPTEs in the TDP MMU's zap_gfn_range(), and rename various
> functions accordingly. When removing mappings for functional correctness
> (except for the stupid VFIO GPU passthrough memslots bug), zapping the
> leaf SPTEs is sufficient as the paging structures themselves do not point
> at guest memory and do not directly impact the final translation (in the
> TDP MMU).
>
> Note, this aligns the TDP MMU with the legacy/full MMU, which zaps only
> the rmaps, a.k.a. leaf SPTEs, in kvm_zap_gfn_range() and
> kvm_unmap_gfn_range().
>
> Signed-off-by: Sean Christopherson <seanjc@...gle.com>
> Reviewed-by: Ben Gardon <bgardon@...gle.com>
> Message-Id: <20220226001546.360188-18-seanjc@...gle.com>
> Signed-off-by: Paolo Bonzini <pbonzini@...hat.com>
> ---
> arch/x86/kvm/mmu/mmu.c | 4 ++--
> arch/x86/kvm/mmu/tdp_mmu.c | 41 ++++++++++----------------------------
> arch/x86/kvm/mmu/tdp_mmu.h | 8 +-------
> 3 files changed, 14 insertions(+), 39 deletions(-)
>
> diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
> index 8408d7db8d2a..febdcaaa7b94 100644
> --- a/arch/x86/kvm/mmu/mmu.c
> +++ b/arch/x86/kvm/mmu/mmu.c
> @@ -5834,8 +5834,8 @@ void kvm_zap_gfn_range(struct kvm *kvm, gfn_t gfn_start, gfn_t gfn_end)
>
> if (is_tdp_mmu_enabled(kvm)) {
> for (i = 0; i < KVM_ADDRESS_SPACE_NUM; i++)
> - flush = kvm_tdp_mmu_zap_gfn_range(kvm, i, gfn_start,
> - gfn_end, flush);
> + flush = kvm_tdp_mmu_zap_leafs(kvm, i, gfn_start,
> + gfn_end, true, flush);
> }
>
> if (flush)
> diff --git a/arch/x86/kvm/mmu/tdp_mmu.c b/arch/x86/kvm/mmu/tdp_mmu.c
> index f3939ce4a115..c71debdbc732 100644
> --- a/arch/x86/kvm/mmu/tdp_mmu.c
> +++ b/arch/x86/kvm/mmu/tdp_mmu.c
> @@ -834,10 +834,8 @@ bool kvm_tdp_mmu_zap_sp(struct kvm *kvm, struct kvm_mmu_page *sp)
> }
>
> /*
> - * Tears down the mappings for the range of gfns, [start, end), and frees the
> - * non-root pages mapping GFNs strictly within that range. Returns true if
> - * SPTEs have been cleared and a TLB flush is needed before releasing the
> - * MMU lock.
> + * Zap leafs SPTEs for the range of gfns, [start, end). Returns true if SPTEs
> + * have been cleared and a TLB flush is needed before releasing the MMU lock.
I think the original code does not _over_ zapping. But the new version
does. Will that have some side effects? In particular, if the range is
within a huge page (or HugeTLB page of various sizes), then we choose to
zap it even if it is more than the range.
Regardless of side effect, I think we probably should mention that in
the comments?
> *
> * If can_yield is true, will release the MMU lock and reschedule if the
> * scheduler needs the CPU or there is contention on the MMU lock. If this
> @@ -845,42 +843,25 @@ bool kvm_tdp_mmu_zap_sp(struct kvm *kvm, struct kvm_mmu_page *sp)
> * the caller must ensure it does not supply too large a GFN range, or the
> * operation can cause a soft lockup.
> */
> -static bool zap_gfn_range(struct kvm *kvm, struct kvm_mmu_page *root,
> - gfn_t start, gfn_t end, bool can_yield, bool flush)
> +static bool tdp_mmu_zap_leafs(struct kvm *kvm, struct kvm_mmu_page *root,
> + gfn_t start, gfn_t end, bool can_yield, bool flush)
> {
> - bool zap_all = (start == 0 && end >= tdp_mmu_max_gfn_host());
> struct tdp_iter iter;
>
> - /*
> - * No need to try to step down in the iterator when zapping all SPTEs,
> - * zapping the top-level non-leaf SPTEs will recurse on their children.
> - */
> - int min_level = zap_all ? root->role.level : PG_LEVEL_4K;
> -
> end = min(end, tdp_mmu_max_gfn_host());
>
> lockdep_assert_held_write(&kvm->mmu_lock);
>
> rcu_read_lock();
>
> - for_each_tdp_pte_min_level(iter, root, min_level, start, end) {
> + for_each_tdp_pte_min_level(iter, root, PG_LEVEL_4K, start, end) {
> if (can_yield &&
> tdp_mmu_iter_cond_resched(kvm, &iter, flush, false)) {
> flush = false;
> continue;
> }
>
> - if (!is_shadow_present_pte(iter.old_spte))
> - continue;
> -
> - /*
> - * If this is a non-last-level SPTE that covers a larger range
> - * than should be zapped, continue, and zap the mappings at a
> - * lower level, except when zapping all SPTEs.
> - */
> - if (!zap_all &&
> - (iter.gfn < start ||
> - iter.gfn + KVM_PAGES_PER_HPAGE(iter.level) > end) &&
> + if (!is_shadow_present_pte(iter.old_spte) ||
> !is_last_spte(iter.old_spte, iter.level))
> continue;
>
> @@ -898,13 +879,13 @@ static bool zap_gfn_range(struct kvm *kvm, struct kvm_mmu_page *root,
> * SPTEs have been cleared and a TLB flush is needed before releasing the
> * MMU lock.
> */
> -bool __kvm_tdp_mmu_zap_gfn_range(struct kvm *kvm, int as_id, gfn_t start,
> - gfn_t end, bool can_yield, bool flush)
> +bool kvm_tdp_mmu_zap_leafs(struct kvm *kvm, int as_id, gfn_t start, gfn_t end,
> + bool can_yield, bool flush)
> {
> struct kvm_mmu_page *root;
>
> for_each_tdp_mmu_root_yield_safe(kvm, root, as_id)
> - flush = zap_gfn_range(kvm, root, start, end, can_yield, flush);
> + flush = tdp_mmu_zap_leafs(kvm, root, start, end, can_yield, false);
>
> return flush;
> }
> @@ -1202,8 +1183,8 @@ int kvm_tdp_mmu_map(struct kvm_vcpu *vcpu, struct kvm_page_fault *fault)
> bool kvm_tdp_mmu_unmap_gfn_range(struct kvm *kvm, struct kvm_gfn_range *range,
> bool flush)
> {
> - return __kvm_tdp_mmu_zap_gfn_range(kvm, range->slot->as_id, range->start,
> - range->end, range->may_block, flush);
> + return kvm_tdp_mmu_zap_leafs(kvm, range->slot->as_id, range->start,
> + range->end, range->may_block, flush);
> }
>
> typedef bool (*tdp_handler_t)(struct kvm *kvm, struct tdp_iter *iter,
> diff --git a/arch/x86/kvm/mmu/tdp_mmu.h b/arch/x86/kvm/mmu/tdp_mmu.h
> index 5e5ef2576c81..54bc8118c40a 100644
> --- a/arch/x86/kvm/mmu/tdp_mmu.h
> +++ b/arch/x86/kvm/mmu/tdp_mmu.h
> @@ -15,14 +15,8 @@ __must_check static inline bool kvm_tdp_mmu_get_root(struct kvm_mmu_page *root)
> void kvm_tdp_mmu_put_root(struct kvm *kvm, struct kvm_mmu_page *root,
> bool shared);
>
> -bool __kvm_tdp_mmu_zap_gfn_range(struct kvm *kvm, int as_id, gfn_t start,
> +bool kvm_tdp_mmu_zap_leafs(struct kvm *kvm, int as_id, gfn_t start,
> gfn_t end, bool can_yield, bool flush);
> -static inline bool kvm_tdp_mmu_zap_gfn_range(struct kvm *kvm, int as_id,
> - gfn_t start, gfn_t end, bool flush)
> -{
> - return __kvm_tdp_mmu_zap_gfn_range(kvm, as_id, start, end, true, flush);
> -}
> -
> bool kvm_tdp_mmu_zap_sp(struct kvm *kvm, struct kvm_mmu_page *sp);
> void kvm_tdp_mmu_zap_all(struct kvm *kvm);
> void kvm_tdp_mmu_invalidate_all_roots(struct kvm *kvm);
> --
> 2.31.1
>
>
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