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Fri, 26 Jul 2024 16:52:56 -0700 (PDT) Reply-To: Sean Christopherson Date: Fri, 26 Jul 2024 16:51:18 -0700 In-Reply-To: <20240726235234.228822-1-seanjc@google.com> Precedence: bulk X-Mailing-List: kvm@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: Mime-Version: 1.0 References: <20240726235234.228822-1-seanjc@google.com> X-Mailer: git-send-email 2.46.0.rc1.232.g9752f9e123-goog Message-ID: <20240726235234.228822-10-seanjc@google.com> Subject: [PATCH v12 09/84] KVM: x86/mmu: Don't force flush if SPTE update clears Accessed bit From: Sean Christopherson To: Paolo Bonzini , Marc Zyngier , Oliver Upton , Tianrui Zhao , Bibo Mao , Huacai Chen , Michael Ellerman , Anup Patel , Paul Walmsley , Palmer Dabbelt , Albert Ou , Christian Borntraeger , Janosch Frank , Claudio Imbrenda , Sean Christopherson Cc: kvm@vger.kernel.org, linux-arm-kernel@lists.infradead.org, kvmarm@lists.linux.dev, loongarch@lists.linux.dev, linux-mips@vger.kernel.org, linuxppc-dev@lists.ozlabs.org, kvm-riscv@lists.infradead.org, linux-riscv@lists.infradead.org, linux-kernel@vger.kernel.org, David Matlack , David Stevens Don't force a TLB flush if mmu_spte_update() clears Accessed bit, as access tracking tolerates false negatives, as evidenced by the mmu_notifier hooks that explicit test and age SPTEs without doing a TLB flush. In practice, this is very nearly a nop. spte_write_protect() and spte_clear_dirty() never clear the Accessed bit. make_spte() always sets the Accessed bit for !prefetch scenarios. FNAME(sync_spte) only sets SPTE if the protection bits are changing, i.e. if a flush will be needed regardless of the Accessed bits. And FNAME(pte_prefetch) sets SPTE if and only if the old SPTE is !PRESENT. That leaves kvm_arch_async_page_ready() as the one path that will generate a !ACCESSED SPTE *and* overwrite a PRESENT SPTE. And that's very arguably a bug, as clobbering a valid SPTE in that case is nonsensical. Signed-off-by: Sean Christopherson --- arch/x86/kvm/mmu/mmu.c | 31 +++++++++---------------------- 1 file changed, 9 insertions(+), 22 deletions(-) diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c index 58b70328b20c..b7642f1f993f 100644 --- a/arch/x86/kvm/mmu/mmu.c +++ b/arch/x86/kvm/mmu/mmu.c @@ -518,37 +518,24 @@ static u64 mmu_spte_update_no_track(u64 *sptep, u64 new_spte) * TLBs must be flushed. Otherwise rmap_write_protect will find a read-only * spte, even though the writable spte might be cached on a CPU's TLB. * + * Remote TLBs also need to be flushed if the Dirty bit is cleared, as false + * negatives are not acceptable, e.g. if KVM is using D-bit based PML on VMX. + * + * Don't flush if the Accessed bit is cleared, as access tracking tolerates + * false negatives, and the one path that does care about TLB flushes, + * kvm_mmu_notifier_clear_flush_young(), uses mmu_spte_update_no_track(). + * * Returns true if the TLB needs to be flushed */ static bool mmu_spte_update(u64 *sptep, u64 new_spte) { - bool flush = false; u64 old_spte = mmu_spte_update_no_track(sptep, new_spte); if (!is_shadow_present_pte(old_spte)) return false; - /* - * For the spte updated out of mmu-lock is safe, since - * we always atomically update it, see the comments in - * spte_has_volatile_bits(). - */ - if (is_mmu_writable_spte(old_spte) && - !is_writable_pte(new_spte)) - flush = true; - - /* - * Flush TLB when accessed/dirty states are changed in the page tables, - * to guarantee consistency between TLB and page tables. - */ - - if (is_accessed_spte(old_spte) && !is_accessed_spte(new_spte)) - flush = true; - - if (is_dirty_spte(old_spte) && !is_dirty_spte(new_spte)) - flush = true; - - return flush; + return (is_mmu_writable_spte(old_spte) && !is_writable_pte(new_spte)) || + (is_dirty_spte(old_spte) && !is_dirty_spte(new_spte)); } /*