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

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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>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
This commit is contained in:
Paolo Bonzini 2022-06-07 10:09:03 -04:00 committed by Greg Kroah-Hartman
parent a88638a954
commit 1eb3e32de7
4 changed files with 28 additions and 1 deletions
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3
arch/x86/include/asm/kvm_host.h
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@ -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;

3
arch/x86/kvm/svm.c
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@ -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)

1
arch/x86/kvm/vmx/vmx.c
View File

@ -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);

22
arch/x86/kvm/x86.c
View File

@ -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 {