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

[Sasha Levin <sashal@kernel.org>]

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