From: Paolo Bonzini <pbonzini@xxxxxxxxxx>
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@xxxxxxxxxx>
Signed-off-by: Paolo Bonzini <pbonzini@xxxxxxxxxx>
[risbhat@xxxxxxxxxx: Use VCPU_STAT to expose preemption_reported and
preemption_other debugfs entries.]
Signed-off-by: Rishabh Bhatnagar <risbhat@xxxxxxxxxx>
---
arch/x86/include/asm/kvm_host.h | 3 +++
arch/x86/kvm/svm/svm.c | 2 ++
arch/x86/kvm/vmx/vmx.c | 1 +
arch/x86/kvm/x86.c | 22 ++++++++++++++++++++++
4 files changed, 28 insertions(+)
diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
index 2b35f8139f15..25b720304640 100644
--- a/arch/x86/include/asm/kvm_host.h
+++ b/arch/x86/include/asm/kvm_host.h
@@ -553,6 +553,7 @@ struct kvm_vcpu_arch {
u64 ia32_misc_enable_msr;
u64 smbase;
u64 smi_count;
+ bool at_instruction_boundary;
bool tpr_access_reporting;
bool xsaves_enabled;
u64 ia32_xss;
@@ -1061,6 +1062,8 @@ struct kvm_vcpu_stat {
u64 req_event;
u64 halt_poll_success_ns;
u64 halt_poll_fail_ns;
+ u64 preemption_reported;
+ u64 preemption_other;
};
struct x86_instruction_info;
diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
index 442705517caf..f3b7a6a82b07 100644
--- a/arch/x86/kvm/svm/svm.c
+++ b/arch/x86/kvm/svm/svm.c
@@ -3991,6 +3991,8 @@ 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 b33d0f283d4f..b8a6ab210c4e 100644
--- a/arch/x86/kvm/vmx/vmx.c
+++ b/arch/x86/kvm/vmx/vmx.c
@@ -6496,6 +6496,7 @@ static void handle_external_interrupt_irqoff(struct kvm_vcpu *vcpu)
return;
handle_interrupt_nmi_irqoff(vcpu, gate_offset(desc));
+ vcpu->arch.at_instruction_boundary = true;
}
static void vmx_handle_exit_irqoff(struct kvm_vcpu *vcpu)
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 0df41be32314..75494b3c2d1e 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -231,6 +231,8 @@ struct kvm_stats_debugfs_item debugfs_entries[] = {
VCPU_STAT("l1d_flush", l1d_flush),
VCPU_STAT("halt_poll_success_ns", halt_poll_success_ns),
VCPU_STAT("halt_poll_fail_ns", halt_poll_fail_ns),
+ VCPU_STAT("preemption_reported", preemption_reported),
+ VCPU_STAT("preemption_other", preemption_other),
VM_STAT("mmu_shadow_zapped", mmu_shadow_zapped),
VM_STAT("mmu_pte_write", mmu_pte_write),
VM_STAT("mmu_pde_zapped", mmu_pde_zapped),
@@ -4088,6 +4090,19 @@ static void kvm_steal_time_set_preempted(struct kvm_vcpu *vcpu)
struct kvm_memslots *slots;
static const u8 preempted = KVM_VCPU_PREEMPTED;
+ /*
+ * 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;
@@ -9300,6 +9315,13 @@ 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 {
--
2.37.1
