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Mon, 24 Feb 2025 15:55:54 -0800 (PST) Reply-To: Sean Christopherson Date: Mon, 24 Feb 2025 15:55:40 -0800 In-Reply-To: <20250224235542.2562848-1-seanjc@google.com> X-Mailing-List: linuxppc-dev@lists.ozlabs.org List-Id: List-Help: List-Owner: List-Post: List-Archive: , List-Subscribe: , , List-Unsubscribe: Mime-Version: 1.0 References: <20250224235542.2562848-1-seanjc@google.com> X-Mailer: git-send-email 2.48.1.658.g4767266eb4-goog Message-ID: <20250224235542.2562848-6-seanjc@google.com> Subject: [PATCH 5/7] KVM: x86: Unload MMUs during vCPU destruction, not before From: Sean Christopherson To: Marc Zyngier , Oliver Upton , Tianrui Zhao , Bibo Mao , Huacai Chen , Madhavan Srinivasan , Anup Patel , Paul Walmsley , Palmer Dabbelt , Albert Ou , Christian Borntraeger , Janosch Frank , Claudio Imbrenda , Sean Christopherson , Paolo Bonzini Cc: linux-arm-kernel@lists.infradead.org, kvmarm@lists.linux.dev, kvm@vger.kernel.org, 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, Aaron Lewis , Jim Mattson , Yan Zhao , Rick P Edgecombe , Kai Huang , Isaku Yamahata Content-Type: text/plain; charset="UTF-8" When destroying a VM, unload a vCPU's MMUs as part of normal vCPU freeing, instead of as a separate prepratory action. Unloading MMUs ahead of time is a holdover from commit 7b53aa565084 ("KVM: Fix vcpu freeing for guest smp"), which "fixed" a rather egregious flaw where KVM would attempt to free *all* MMU pages when destroying a vCPU. At the time, KVM would spin on all MMU pages in a VM when free a single vCPU, and so would hang due to the way KVM pins and zaps root pages (roots are invalidated but not freed if they are pinned by a vCPU). static void free_mmu_pages(struct kvm_vcpu *vcpu) { struct kvm_mmu_page *page; while (!list_empty(&vcpu->kvm->active_mmu_pages)) { page = container_of(vcpu->kvm->active_mmu_pages.next, struct kvm_mmu_page, link); kvm_mmu_zap_page(vcpu->kvm, page); } free_page((unsigned long)vcpu->mmu.pae_root); } Now that KVM doesn't try to free all MMU pages when destroying a single vCPU, there's no need to unpin roots prior to destroying a vCPU. Note! While KVM mostly destroys all MMUs before calling kvm_arch_destroy_vm() (see commit f00be0cae4e6 ("KVM: MMU: do not free active mmu pages in free_mmu_pages()")), unpinning MMU roots during vCPU destruction will unfortunately trigger remote TLB flushes, i.e. will try to send requests to all vCPUs. Happily, thanks to commit 27592ae8dbe4 ("KVM: Move wiping of the kvm->vcpus array to common code"), that's a non-issue as freed vCPUs are naturally skipped by xa_for_each_range(), i.e. by kvm_for_each_vcpu(). Prior to that commit, KVM x86 rather stupidly freed vCPUs one-by-one, and _then_ nullified them, one-by-one. I.e. triggering a VM-wide request would hit a use-after-free. Signed-off-by: Sean Christopherson --- arch/x86/kvm/x86.c | 15 +++------------ 1 file changed, 3 insertions(+), 12 deletions(-) diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c index 9978ed4c0917..a61dbd1f0d01 100644 --- a/arch/x86/kvm/x86.c +++ b/arch/x86/kvm/x86.c @@ -12374,6 +12374,9 @@ void kvm_arch_vcpu_destroy(struct kvm_vcpu *vcpu) { int idx; + kvm_clear_async_pf_completion_queue(vcpu); + kvm_mmu_unload(vcpu); + kvmclock_reset(vcpu); kvm_x86_call(vcpu_free)(vcpu); @@ -12767,17 +12770,6 @@ int kvm_arch_init_vm(struct kvm *kvm, unsigned long type) return ret; } -static void kvm_unload_vcpu_mmus(struct kvm *kvm) -{ - unsigned long i; - struct kvm_vcpu *vcpu; - - kvm_for_each_vcpu(i, vcpu, kvm) { - kvm_clear_async_pf_completion_queue(vcpu); - kvm_mmu_unload(vcpu); - } -} - void kvm_arch_sync_events(struct kvm *kvm) { cancel_delayed_work_sync(&kvm->arch.kvmclock_sync_work); @@ -12882,7 +12874,6 @@ void kvm_arch_destroy_vm(struct kvm *kvm) __x86_set_memory_region(kvm, TSS_PRIVATE_MEMSLOT, 0, 0); mutex_unlock(&kvm->slots_lock); } - kvm_unload_vcpu_mmus(kvm); kvm_destroy_vcpus(kvm); kvm_x86_call(vm_destroy)(kvm); kvm_free_msr_filter(srcu_dereference_check(kvm->arch.msr_filter, &kvm->srcu, 1)); -- 2.48.1.658.g4767266eb4-goog