This is a note to let you know that I've just added the patch titled
KVM: x86: do not report a vCPU as preempted outside instruction boundaries
to the 5.10-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary
The filename of the patch is:
kvm-x86-do-not-report-a-vcpu-as-preempted-outside-instruction-boundaries.patch
and it can be found in the queue-5.10 subdirectory.
If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@xxxxxxxxxxxxxxx> know about it.
>From 6cd88243c7e03845a450795e134b488fc2afb736 Mon Sep 17 00:00:00 2001
From: Paolo Bonzini <pbonzini@xxxxxxxxxx>
Date: Tue, 7 Jun 2022 10:09:03 -0400
Subject: KVM: x86: do not report a vCPU as preempted outside instruction boundaries
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>
[OP: use VCPU_STAT() for debugfs entries]
Signed-off-by: Ovidiu Panait <ovidiu.panait@xxxxxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
---
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(+)
--- 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;
--- a/arch/x86/kvm/svm/svm.c
+++ b/arch/x86/kvm/svm/svm.c
@@ -3983,6 +3983,8 @@ out:
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)
--- a/arch/x86/kvm/vmx/vmx.c
+++ b/arch/x86/kvm/vmx/vmx.c
@@ -6510,6 +6510,7 @@ static void handle_external_interrupt_ir
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)
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -231,6 +231,8 @@ struct kvm_stats_debugfs_item debugfs_en
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),
@@ -4052,6 +4054,19 @@ static void kvm_steal_time_set_preempted
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;
@@ -9357,6 +9372,13 @@ static int vcpu_run(struct kvm_vcpu *vcp
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 {
Patches currently in stable-queue which might be from pbonzini@xxxxxxxxxx are
queue-5.10/kvm-x86-do-not-report-a-vcpu-as-preempted-outside-instruction-boundaries.patch
queue-5.10/kvm-x86-hyper-v-avoid-calling-kvm_make_vcpus_request_mask-with-vcpu_mask-null.patch
queue-5.10/kvm-nvmx-emulate-nops-in-l2-and-pause-if-it-s-not-intercepted.patch
