From 60d93c27859fb3ec5b5a689e24de303f166477d0 Mon Sep 17 00:00:00 2001 From: Sean Christopherson Date: Fri, 4 Sep 2026 10:06:39 -0700 Subject: [PATCH] KVM: SVM: Preserve TLB control (i.e. pending TLB flush) on failed VMRUN Don't reset the VMCB's TLB control back to "do nothing" on a failed VMRUN, as empirical testing shows that the CPU performs the requested TLB flush if and only if VMRUN is successful, i.e. clearing TLB control on a failed VMRUN effectively drops a TLB flush. Explicitly track the need to flush all ASIDs on a per-CPU basis, as the ASID reuse condition is tied to the pCPU, not to the vCPU. As a bonus, this also obviates the need to avoid clobbering FLUSH_ALL_ASID with TLB_CONTROL_FLUSH_ASID, e.g. in svm_flush_tlb_asid(). Deliberately don't bother saving/restoring the "old" tlb_ctl on failure, in quotes because it's not exactly the old tlb_ctl, it's the tlb_ctl from after pre_svm_run(), but before updating tlb_ctl for flush_all_asids. If VMRUN fails and TLB_CONTROL_FLUSH_ALL_ASID is forced, then the next successful run of the VMCB *may* unnecessarily flush all ASIDs, which strictly speaking could result in noisy neighbor issues. However, the fact that new_asid() is already guest-triggerable, because of KVM's flawed behavior of clearing the ASID on emulated INIT, means that a guest can already trigger a flush of all ASIDs at roughly the same rate. And once KVM stops clobbering the ASID on emulated INIT, *or* assigns a static ASID to each vCPU, this flaw goes away. Fixes: 38e5e92fe8c0 ("KVM: SVM: Implement Flush-By-Asid feature") Cc: stable@vger.kernel.org Reported-by: Stefan Teodorescu Suggested-by: Yosry Ahmed Cc: Tom Lendacky Cc: Jim Mattson Signed-off-by: Sean Christopherson Message-ID: <20260904170642.3291466-2-seanjc@google.com> Signed-off-by: Paolo Bonzini --- arch/x86/kvm/svm/svm.c | 13 ++++++++++--- arch/x86/kvm/svm/svm.h | 1 + 2 files changed, 11 insertions(+), 3 deletions(-) diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c index 7d59d301e1e5..afbaaaab84ed 100644 --- a/arch/x86/kvm/svm/svm.c +++ b/arch/x86/kvm/svm/svm.c @@ -1902,8 +1902,7 @@ static void new_asid(struct vcpu_svm *svm, struct svm_cpu_data *sd) if (sd->next_asid > sd->max_asid) { ++sd->asid_generation; sd->next_asid = sd->min_asid; - svm->vmcb->control.tlb_ctl = TLB_CONTROL_FLUSH_ALL_ASID; - vmcb_mark_dirty(svm->vmcb, VMCB_ASID); + sd->flush_all_asids = true; } svm->current_vmcb->asid_generation = sd->asid_generation; @@ -4528,6 +4527,11 @@ static __no_kcsan fastpath_t svm_vcpu_run(struct kvm_vcpu *vcpu, u64 run_flags) svm->vmcb->control.asid = svm->asid; vmcb_mark_dirty(svm->vmcb, VMCB_ASID); } + if (this_cpu_ptr(&svm_data)->flush_all_asids) { + svm->vmcb->control.tlb_ctl = TLB_CONTROL_FLUSH_ALL_ASID; + vmcb_mark_dirty(svm->vmcb, VMCB_ASID); + } + svm->vmcb->save.cr2 = vcpu->arch.cr2; if (guest_cpu_cap_has(vcpu, X86_FEATURE_ERAPS) && @@ -4618,7 +4622,10 @@ static __no_kcsan fastpath_t svm_vcpu_run(struct kvm_vcpu *vcpu, u64 run_flags) vcpu->arch.nested_run_pending = 0; } - svm->vmcb->control.tlb_ctl = TLB_CONTROL_DO_NOTHING; + if (!svm_is_vmrun_failure(svm->vmcb->control.exit_code)) { + this_cpu_ptr(&svm_data)->flush_all_asids = false; + svm->vmcb->control.tlb_ctl = TLB_CONTROL_DO_NOTHING; + } /* * Unconditionally mask off the CLEAR_RAP bit, the AND is just as cheap diff --git a/arch/x86/kvm/svm/svm.h b/arch/x86/kvm/svm/svm.h index e958943b8162..84f19026d3e8 100644 --- a/arch/x86/kvm/svm/svm.h +++ b/arch/x86/kvm/svm/svm.h @@ -376,6 +376,7 @@ struct svm_cpu_data { u32 next_asid; u32 min_asid; + bool flush_all_asids; bool bp_spec_reduce_set; struct vmcb *save_area;