From: Paolo Bonzini pbonzini@redhat.com

commit 6cd88243c7e03845a450795e134b488fc2afb736 upstream.

If a vCPU is outside guest mode and is scheduled out, it might be in the process of making a memory access. A problem occurs if another vCPU uses the PV TLB flush feature during the period when the vCPU is scheduled out, and a virtual address has already been translated but has not yet been accessed, because this is equivalent to using a stale TLB entry.

To avoid this, only report a vCPU as preempted if sure that the guest is at an instruction boundary. A rescheduling request will be delivered to the host physical CPU as an external interrupt, so for simplicity consider any vmexit *not* instruction boundary except for external interrupts.

It would in principle be okay to report the vCPU as preempted also if it is sleeping in kvm_vcpu_block(): a TLB flush IPI will incur the vmentry/vmexit overhead unnecessarily, and optimistic spinning is also unlikely to succeed. However, leave it for later because right now kvm_vcpu_check_block() is doing memory accesses. Even though the TLB flush issue only applies to virtual memory address, it's very much preferrable to be conservative.

Reported-by: Jann Horn jannh@google.com Signed-off-by: Paolo Bonzini pbonzini@redhat.com [OP: use VCPU_STAT() for debugfs entries] Signed-off-by: Ovidiu Panait ovidiu.panait@windriver.com --- arch/x86/include/asm/kvm_host.h | 3 +++ arch/x86/kvm/svm.c | 3 ++- arch/x86/kvm/vmx/vmx.c | 1 + arch/x86/kvm/x86.c | 22 ++++++++++++++++++++++ 4 files changed, 28 insertions(+), 1 deletion(-)

diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h index 4bc476d7fa6c..80239c84b4dd 100644 --- a/arch/x86/include/asm/kvm_host.h +++ b/arch/x86/include/asm/kvm_host.h @@ -563,6 +563,7 @@ struct kvm_vcpu_arch { u64 ia32_misc_enable_msr; u64 smbase; u64 smi_count; + bool at_instruction_boundary; bool tpr_access_reporting; u64 ia32_xss; u64 microcode_version; @@ -981,6 +982,8 @@ struct kvm_vcpu_stat { u64 irq_injections; u64 nmi_injections; u64 req_event; + u64 preemption_reported; + u64 preemption_other; };

struct x86_instruction_info; diff --git a/arch/x86/kvm/svm.c b/arch/x86/kvm/svm.c index c5a9de8d0725..e9444e202c33 100644 --- a/arch/x86/kvm/svm.c +++ b/arch/x86/kvm/svm.c @@ -6246,7 +6246,8 @@ static int svm_check_intercept(struct kvm_vcpu *vcpu,

static void svm_handle_exit_irqoff(struct kvm_vcpu *vcpu) { - + if (to_svm(vcpu)->vmcb->control.exit_code == SVM_EXIT_INTR) + vcpu->arch.at_instruction_boundary = true; }

static void svm_sched_in(struct kvm_vcpu *vcpu, int cpu) diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c index 9bd08d264603..c93070829790 100644 --- a/arch/x86/kvm/vmx/vmx.c +++ b/arch/x86/kvm/vmx/vmx.c @@ -6358,6 +6358,7 @@ static void handle_external_interrupt_irqoff(struct kvm_vcpu *vcpu) );

kvm_after_interrupt(vcpu); + vcpu->arch.at_instruction_boundary = true; } STACK_FRAME_NON_STANDARD(handle_external_interrupt_irqoff);

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c index f5e9590a8f31..d152afdfa8b4 100644 --- a/arch/x86/kvm/x86.c +++ b/arch/x86/kvm/x86.c @@ -207,6 +207,8 @@ struct kvm_stats_debugfs_item debugfs_entries[] = { { "nmi_injections", VCPU_STAT(nmi_injections) }, { "req_event", VCPU_STAT(req_event) }, { "l1d_flush", VCPU_STAT(l1d_flush) }, + { "preemption_reported", VCPU_STAT(preemption_reported) }, + { "preemption_other", VCPU_STAT(preemption_other) }, { "mmu_shadow_zapped", VM_STAT(mmu_shadow_zapped) }, { "mmu_pte_write", VM_STAT(mmu_pte_write) }, { "mmu_pde_zapped", VM_STAT(mmu_pde_zapped) }, @@ -3562,6 +3564,19 @@ static void kvm_steal_time_set_preempted(struct kvm_vcpu *vcpu) struct kvm_host_map map; struct kvm_steal_time *st;

+ /* + * The vCPU can be marked preempted if and only if the VM-Exit was on + * an instruction boundary and will not trigger guest emulation of any + * kind (see vcpu_run). Vendor specific code controls (conservatively) + * when this is true, for example allowing the vCPU to be marked + * preempted if and only if the VM-Exit was due to a host interrupt. + */ + if (!vcpu->arch.at_instruction_boundary) { + vcpu->stat.preemption_other++; + return; + } + + vcpu->stat.preemption_reported++; if (!(vcpu->arch.st.msr_val & KVM_MSR_ENABLED)) return;

@@ -8446,6 +8461,13 @@ static int vcpu_run(struct kvm_vcpu *vcpu) vcpu->arch.l1tf_flush_l1d = true;

for (;;) { + /* + * If another guest vCPU requests a PV TLB flush in the middle + * of instruction emulation, the rest of the emulation could + * use a stale page translation. Assume that any code after + * this point can start executing an instruction. + */ + vcpu->arch.at_instruction_boundary = false; if (kvm_vcpu_running(vcpu)) { r = vcpu_enter_guest(vcpu); } else {