Commit 6cd88243 authored by Paolo Bonzini's avatar Paolo Bonzini

KVM: x86: do not report a vCPU as preempted outside instruction boundaries

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: default avatarJann Horn <jannh@google.com>
Signed-off-by: default avatarPaolo Bonzini <pbonzini@redhat.com>
parent 54aa83c9
......@@ -653,6 +653,7 @@ 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;
bool xfd_no_write_intercept;
......@@ -1300,6 +1301,8 @@ struct kvm_vcpu_stat {
u64 nested_run;
u64 directed_yield_attempted;
u64 directed_yield_successful;
u64 preemption_reported;
u64 preemption_other;
u64 guest_mode;
};
......
......@@ -4263,6 +4263,8 @@ 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)
......
......@@ -6547,6 +6547,7 @@ 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)
......
......@@ -296,6 +296,8 @@ const struct _kvm_stats_desc kvm_vcpu_stats_desc[] = {
STATS_DESC_COUNTER(VCPU, nested_run),
STATS_DESC_COUNTER(VCPU, directed_yield_attempted),
STATS_DESC_COUNTER(VCPU, directed_yield_successful),
STATS_DESC_COUNTER(VCPU, preemption_reported),
STATS_DESC_COUNTER(VCPU, preemption_other),
STATS_DESC_ICOUNTER(VCPU, guest_mode)
};
......@@ -4625,6 +4627,19 @@ 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;
......@@ -10424,6 +10439,13 @@ 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 {
......
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