| Message ID | 20250303052227.523411-1-zijie.wei@linux.alibaba.com (mailing list archive) |
|---|---|
| State | New |
| Headers | show |
| Series | [v4] KVM: x86: ioapic: Optimize EOI handling to reduce unnecessary VM exits | expand |
Several minor comments. No need to post v5, I'll do so today as a small series with preparatory patches to refactor and deduplicate the userspace vs. in-kernel logic. On Mon, Mar 03, 2025, weizijie wrote: > Configuring IOAPIC routed interrupts triggers KVM to rescan all vCPU's > ioapic_handled_vectors which is used to control which vectors need to > trigger EOI-induced VMEXITs. If any interrupt is already in service on > some vCPU using some vector when the IOAPIC is being rescanned, the > vector is set to vCPU's ioapic_handled_vectors. If the vector is then > reused by other interrupts, each of them will cause a VMEXIT even it is > unnecessary. W/o further IOAPIC rescan, the vector remains set, and this > issue persists, impacting guest's interrupt performance. > > Both > > commit db2bdcbbbd32 ("KVM: x86: fix edge EOI and IOAPIC reconfig race") > > and > > commit 0fc5a36dd6b3 ("KVM: x86: ioapic: Fix level-triggered EOI and > IOAPIC reconfigure race") > > mentioned this issue, but it was considered as "rare" thus was not > addressed. However in real environment this issue can actually happen > in a well-behaved guest. > > Simple Fix Proposal: > A straightforward solution is to record highest in-service IRQ that > is pending at the time of the last scan. Then, upon the next guest > exit, do a full KVM_REQ_SCAN_IOAPIC. This ensures that a re-scan of > the ioapic occurs only when the recorded vector is EOI'd, and > subsequently, the extra bits in the eoi_exit_bitmap are cleared, > avoiding unnecessary VM exits. Write changelogs as "commands", i.e. state what changes are actually being made, as opposed to passively describing a proposed/possible change. > Co-developed-by: xuyun <xuyun_xy.xy@linux.alibaba.com> > Signed-off-by: xuyun <xuyun_xy.xy@linux.alibaba.com> > Signed-off-by: weizijie <zijie.wei@linux.alibaba.com> > Reviewed-by: Kai Huang <kai.huang@intel.com> > --- > arch/x86/include/asm/kvm_host.h | 1 + > arch/x86/kvm/ioapic.c | 10 ++++++++-- > arch/x86/kvm/irq_comm.c | 9 +++++++-- > arch/x86/kvm/lapic.c | 13 +++++++++++++ > 4 files changed, 29 insertions(+), 4 deletions(-) > > diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h > index 0b7af5902ff7..8c50e7b4a96f 100644 > --- a/arch/x86/include/asm/kvm_host.h > +++ b/arch/x86/include/asm/kvm_host.h > @@ -1062,6 +1062,7 @@ struct kvm_vcpu_arch { > #if IS_ENABLED(CONFIG_HYPERV) > hpa_t hv_root_tdp; > #endif > + u8 last_pending_vector; To be consistent with how KVM handles IRQs, this should probably be an "int" with -1 as the "no pending EOI" value. I also think we should go with a verbose name to try and precisely capture what this field tracks, e.g. highest_stale_pending_ioapic_eoi. It's abusrdly long, but with massaging and refactoring the line lengths are a non-issue, and I want to avoid confusion with pending_ioapic_eoi and highest_isr_cache (and others). > }; > > struct kvm_lpage_info { > diff --git a/arch/x86/kvm/ioapic.c b/arch/x86/kvm/ioapic.c > index 995eb5054360..40252a800897 100644 > --- a/arch/x86/kvm/ioapic.c > +++ b/arch/x86/kvm/ioapic.c > @@ -297,10 +297,16 @@ void kvm_ioapic_scan_entry(struct kvm_vcpu *vcpu, ulong *ioapic_handled_vectors) > u16 dm = kvm_lapic_irq_dest_mode(!!e->fields.dest_mode); > > if (kvm_apic_match_dest(vcpu, NULL, APIC_DEST_NOSHORT, > - e->fields.dest_id, dm) || > - kvm_apic_pending_eoi(vcpu, e->fields.vector)) > + e->fields.dest_id, dm)) > __set_bit(e->fields.vector, > ioapic_handled_vectors); > + else if (kvm_apic_pending_eoi(vcpu, e->fields.vector)) { > + __set_bit(e->fields.vector, > + ioapic_handled_vectors); > + vcpu->arch.last_pending_vector = e->fields.vector > > + vcpu->arch.last_pending_vector ? e->fields.vector : > + vcpu->arch.last_pending_vector; Eh, no need to use a ternary operator, last_pending_vector only needs to be written if it's changing. > } > } > spin_unlock(&ioapic->lock); > diff --git a/arch/x86/kvm/irq_comm.c b/arch/x86/kvm/irq_comm.c > index 8136695f7b96..1d23c52576e1 100644 > --- a/arch/x86/kvm/irq_comm.c > +++ b/arch/x86/kvm/irq_comm.c > @@ -426,9 +426,14 @@ void kvm_scan_ioapic_routes(struct kvm_vcpu *vcpu, > > if (irq.trig_mode && > (kvm_apic_match_dest(vcpu, NULL, APIC_DEST_NOSHORT, > - irq.dest_id, irq.dest_mode) || > - kvm_apic_pending_eoi(vcpu, irq.vector))) > + irq.dest_id, irq.dest_mode))) > __set_bit(irq.vector, ioapic_handled_vectors); > + else if (kvm_apic_pending_eoi(vcpu, irq.vector)) { > + __set_bit(irq.vector, ioapic_handled_vectors); > + vcpu->arch.last_pending_vector = irq.vector > > + vcpu->arch.last_pending_vector ? irq.vector : > + vcpu->arch.last_pending_vector; This is wrong. Well, maybe not "wrong" per se, but unnecessary. The trig_mode check guards both the "new" and "old" routing cases, i.e. KVM's behavior is to intercept EOIs for in-flight IRQs if and only if the IRQ is level-triggered. This code really needs to be de-duplicated between the userspace and in-kernel I/O APICs. It probably should have been de-duplicated by fceb3a36c29a ("KVM: x86: ioapic: Fix level-triggered EOI and userspace I/OAPIC reconfigure race"), but it's a much more pressing issue now. > + } > } > } > srcu_read_unlock(&kvm->irq_srcu, idx); > diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c > index a009c94c26c2..7c540a0eb340 100644 > --- a/arch/x86/kvm/lapic.c > +++ b/arch/x86/kvm/lapic.c > @@ -1466,6 +1466,19 @@ static void kvm_ioapic_send_eoi(struct kvm_lapic *apic, int vector) > if (!kvm_ioapic_handles_vector(apic, vector)) > return; > > + /* > + * When there are instances where ioapic_handled_vectors is > + * set due to pending interrupts, clean up the record and do > + * a full KVM_REQ_SCAN_IOAPIC. > + * This ensures the vector is cleared in the vCPU's ioapic_handled_vectors, > + * if the vector is reused by non-IOAPIC interrupts, avoiding unnecessary > + * EOI-induced VMEXITs for that vector. > + */ > + if (apic->vcpu->arch.last_pending_vector == vector) { > + apic->vcpu->arch.last_pending_vector = 0; I think it makes sense to reset the field when KVM_REQ_SCAN_IOAPIC, mainly so that it's more obviously correct, i.e. so that it's easier to see that the field is reset immediately prior to scanning, along with the bitmap itself. > + kvm_make_request(KVM_REQ_SCAN_IOAPIC, apic->vcpu); > + } > + > /* Request a KVM exit to inform the userspace IOAPIC. */ > if (irqchip_split(apic->vcpu->kvm)) { > apic->vcpu->arch.pending_ioapic_eoi = vector; > -- > 2.43.5 >
diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h index 0b7af5902ff7..8c50e7b4a96f 100644 --- a/arch/x86/include/asm/kvm_host.h +++ b/arch/x86/include/asm/kvm_host.h @@ -1062,6 +1062,7 @@ struct kvm_vcpu_arch { #if IS_ENABLED(CONFIG_HYPERV) hpa_t hv_root_tdp; #endif + u8 last_pending_vector; }; struct kvm_lpage_info { diff --git a/arch/x86/kvm/ioapic.c b/arch/x86/kvm/ioapic.c index 995eb5054360..40252a800897 100644 --- a/arch/x86/kvm/ioapic.c +++ b/arch/x86/kvm/ioapic.c @@ -297,10 +297,16 @@ void kvm_ioapic_scan_entry(struct kvm_vcpu *vcpu, ulong *ioapic_handled_vectors) u16 dm = kvm_lapic_irq_dest_mode(!!e->fields.dest_mode); if (kvm_apic_match_dest(vcpu, NULL, APIC_DEST_NOSHORT, - e->fields.dest_id, dm) || - kvm_apic_pending_eoi(vcpu, e->fields.vector)) + e->fields.dest_id, dm)) __set_bit(e->fields.vector, ioapic_handled_vectors); + else if (kvm_apic_pending_eoi(vcpu, e->fields.vector)) { + __set_bit(e->fields.vector, + ioapic_handled_vectors); + vcpu->arch.last_pending_vector = e->fields.vector > + vcpu->arch.last_pending_vector ? e->fields.vector : + vcpu->arch.last_pending_vector; + } } } spin_unlock(&ioapic->lock); diff --git a/arch/x86/kvm/irq_comm.c b/arch/x86/kvm/irq_comm.c index 8136695f7b96..1d23c52576e1 100644 --- a/arch/x86/kvm/irq_comm.c +++ b/arch/x86/kvm/irq_comm.c @@ -426,9 +426,14 @@ void kvm_scan_ioapic_routes(struct kvm_vcpu *vcpu, if (irq.trig_mode && (kvm_apic_match_dest(vcpu, NULL, APIC_DEST_NOSHORT, - irq.dest_id, irq.dest_mode) || - kvm_apic_pending_eoi(vcpu, irq.vector))) + irq.dest_id, irq.dest_mode))) __set_bit(irq.vector, ioapic_handled_vectors); + else if (kvm_apic_pending_eoi(vcpu, irq.vector)) { + __set_bit(irq.vector, ioapic_handled_vectors); + vcpu->arch.last_pending_vector = irq.vector > + vcpu->arch.last_pending_vector ? irq.vector : + vcpu->arch.last_pending_vector; + } } } srcu_read_unlock(&kvm->irq_srcu, idx); diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c index a009c94c26c2..7c540a0eb340 100644 --- a/arch/x86/kvm/lapic.c +++ b/arch/x86/kvm/lapic.c @@ -1466,6 +1466,19 @@ static void kvm_ioapic_send_eoi(struct kvm_lapic *apic, int vector) if (!kvm_ioapic_handles_vector(apic, vector)) return; + /* + * When there are instances where ioapic_handled_vectors is + * set due to pending interrupts, clean up the record and do + * a full KVM_REQ_SCAN_IOAPIC. + * This ensures the vector is cleared in the vCPU's ioapic_handled_vectors, + * if the vector is reused by non-IOAPIC interrupts, avoiding unnecessary + * EOI-induced VMEXITs for that vector. + */ + if (apic->vcpu->arch.last_pending_vector == vector) { + apic->vcpu->arch.last_pending_vector = 0; + kvm_make_request(KVM_REQ_SCAN_IOAPIC, apic->vcpu); + } + /* Request a KVM exit to inform the userspace IOAPIC. */ if (irqchip_split(apic->vcpu->kvm)) { apic->vcpu->arch.pending_ioapic_eoi = vector;
Configuring IOAPIC routed interrupts triggers KVM to rescan all vCPU's ioapic_handled_vectors which is used to control which vectors need to trigger EOI-induced VMEXITs. If any interrupt is already in service on some vCPU using some vector when the IOAPIC is being rescanned, the vector is set to vCPU's ioapic_handled_vectors. If the vector is then reused by other interrupts, each of them will cause a VMEXIT even it is unnecessary. W/o further IOAPIC rescan, the vector remains set, and this issue persists, impacting guest's interrupt performance. Both commit db2bdcbbbd32 ("KVM: x86: fix edge EOI and IOAPIC reconfig race") and commit 0fc5a36dd6b3 ("KVM: x86: ioapic: Fix level-triggered EOI and IOAPIC reconfigure race") mentioned this issue, but it was considered as "rare" thus was not addressed. However in real environment this issue can actually happen in a well-behaved guest. Simple Fix Proposal: A straightforward solution is to record highest in-service IRQ that is pending at the time of the last scan. Then, upon the next guest exit, do a full KVM_REQ_SCAN_IOAPIC. This ensures that a re-scan of the ioapic occurs only when the recorded vector is EOI'd, and subsequently, the extra bits in the eoi_exit_bitmap are cleared, avoiding unnecessary VM exits. Co-developed-by: xuyun <xuyun_xy.xy@linux.alibaba.com> Signed-off-by: xuyun <xuyun_xy.xy@linux.alibaba.com> Signed-off-by: weizijie <zijie.wei@linux.alibaba.com> Reviewed-by: Kai Huang <kai.huang@intel.com> --- arch/x86/include/asm/kvm_host.h | 1 + arch/x86/kvm/ioapic.c | 10 ++++++++-- arch/x86/kvm/irq_comm.c | 9 +++++++-- arch/x86/kvm/lapic.c | 13 +++++++++++++ 4 files changed, 29 insertions(+), 4 deletions(-)