Re: [PATCH MANUALSEL 5.18 1/6] KVM: x86: do not report a vCPU as preempted outside instruction boundaries

Sasha Levin
2022-06-21T21:27:40Z

Paolo, ping?

On Mon, Jun 13, 2022 at 10:11:10PM -0400, Sasha Levin wrote:
>From: Paolo Bonzini <pbonzini@redhat.com>
>
>[ Upstream commit 6cd88243c7e03845a450795e134b488fc2afb736 ]
>
>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>
>Signed-off-by: Sasha Levin <sashal@kernel.org>
>---
> 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 4ff36610af6a..9fdaa847d4b6 100644
>--- a/arch/x86/include/asm/kvm_host.h
>+++ b/arch/x86/include/asm/kvm_host.h
>@@ -651,6 +651,7 @@ 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;
>@@ -1289,6 +1290,8 @@ struct kvm_vcpu_stat {
> u64 nested_run;
> u64 directed_yield_attempted;
> u64 directed_yield_successful;
>+ u64 preemption_reported;
>+ u64 preemption_other;
> u64 guest_mode;
> };
>
>diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
>index 7e45d03cd018..5842abf1eac4 100644
>--- a/arch/x86/kvm/svm/svm.c
>+++ b/arch/x86/kvm/svm/svm.c
>@@ -4165,6 +4165,8 @@ 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 982df9c000d3..c44f8e1d30c8 100644
>--- a/arch/x86/kvm/vmx/vmx.c
>+++ b/arch/x86/kvm/vmx/vmx.c
>@@ -6549,6 +6549,7 @@ 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 39c571224ac2..36453517e847 100644
>--- a/arch/x86/kvm/x86.c
>+++ b/arch/x86/kvm/x86.c
>@@ -291,6 +291,8 @@ 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)
> };
>
>@@ -4604,6 +4606,19 @@ 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;
>
>@@ -10358,6 +10373,13 @@ 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 {
>--
>2.35.1
>

--
Thanks,
Sasha