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Message-ID: <9553f84e3a4533278e06938a4693991cf23cdfc3.camel@intel.com>
Date: Fri, 28 Feb 2025 12:25:29 +0000
From: "Huang, Kai" <kai.huang@...el.com>
To: "seanjc@...gle.com" <seanjc@...gle.com>, "x86@...nel.org"
<x86@...nel.org>, "dave.hansen@...ux.intel.com"
<dave.hansen@...ux.intel.com>, "bp@...en8.de" <bp@...en8.de>,
"mingo@...hat.com" <mingo@...hat.com>, "tglx@...utronix.de"
<tglx@...utronix.de>, "zijie.wei@...ux.alibaba.com"
<zijie.wei@...ux.alibaba.com>, "pbonzini@...hat.com" <pbonzini@...hat.com>,
"kvm@...r.kernel.org" <kvm@...r.kernel.org>, "hpa@...or.com" <hpa@...or.com>,
"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
CC: "xuyun_xy.xy@...ux.alibaba.com" <xuyun_xy.xy@...ux.alibaba.com>
Subject: Re: [PATCH v3] KVM: x86: ioapic: Optimize EOI handling to reduce
unnecessary VM exits
On Fri, 2025-02-28 at 10:15 +0800, weizijie wrote:
> Address performance issues caused by a vector being reused by a
> non-IOAPIC source.
I saw your reply in v2. Thanks.
Some minor comments below, which may be just nits for Sean/Paolo, so feel free
to add:
Reviewed-by: Kai Huang <kai.huang@...el.com>
>
> Commit 0fc5a36dd6b3
> ("KVM: x86: ioapic: Fix level-triggered EOI and IOAPIC reconfigure race")
> addressed the issues related to EOI and IOAPIC reconfiguration races.
> However, it has introduced some performance concerns:
>
> Configuring IOAPIC interrupts while an interrupt request (IRQ) is
> already in service can unintentionally trigger a VM exit for other
> interrupts that normally do not require one, due to the settings of
> `ioapic_handled_vectors`. If the IOAPIC is not reconfigured during
> runtime, this issue persists, continuing to adversely affect
> performance.
So in short:
The "rare case" mentioned in db2bdcbbbd32 and 0fc5a36dd6b3 is actually
not that rare and can actually happen in the good behaved guest.
The above commit message isn't very clear to me, though. How about below?
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 bit in the eoi_exit_bitmap are cleared,
^
bits
> avoiding unnecessary VM exits.
>
> Co-developed-by: xuyun <xuyun_xy.xy@...ux.alibaba.com>
> Signed-off-by: xuyun <xuyun_xy.xy@...ux.alibaba.com>
> Signed-off-by: weizijie <zijie.wei@...ux.alibaba.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 | 10 ++++++++++
> 4 files changed, 26 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;
> };
>
> 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..5d62ea5f1503 100644
> --- a/arch/x86/kvm/lapic.c
> +++ b/arch/x86/kvm/lapic.c
> @@ -1466,6 +1466,16 @@ 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.
> + */
How about also add below to the comment?
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;
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