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Message-ID: <c220d043-2314-85bb-e99d-dc2c609aa739@loongson.cn>
Date: Tue, 25 Mar 2025 10:13:41 +0800
From: bibo mao <maobibo@...ngson.cn>
To: Paolo Bonzini <pbonzini@...hat.com>,
Sean Christopherson <seanjc@...gle.com>
Cc: Huacai Chen <chenhuacai@...nel.org>,
Tianrui Zhao <zhaotianrui@...ngson.cn>, loongarch@...ts.linux.dev,
linux-kernel@...r.kernel.org, kvm@...r.kernel.org
Subject: Re: [RFC V2] LoongArch: KVM: Handle interrupt early before enabling
irq
Hi Paolo, Sean
This idea comes from x86, do you have any guidance or suggestion about it?
Also I notice that there is such irq_enable()/irq_disable() pair on x86,
I do not know why it is so.
local_irq_enable();
++vcpu->stat.exits;
local_irq_disable();
guest_timing_exit_irqoff();
local_irq_enable();
Regards
Bibo Mao
On 2025/3/11 下午3:47, Bibo Mao wrote:
> If interrupt arrive when vCPU is running, vCPU will exit because of
> interrupt exception. Currently interrupt exception is handled after
> local_irq_enable() is called, and it is handled by host kernel rather
> than KVM hypervisor. It will introduce extra another interrupt
> exception and then host will handle irq.
>
> If KVM hypervisor detect that it is interrupt exception, interrupt
> can be handle early in KVM hypervisor before local_irq_enable() is
> called.
>
> On 3C5000 dual-way machine, there will be 10% -- 15% performance
> improvement with netperf UDP_RR option with 10G ethernet card.
> original with patch improvement
> netperf UDP_RR 7200 8100 +12%
>
> The total performance is low because irqchip is emulated in qemu VMM,
> however from the same testbed, there is performance improvement
> actually.
>
> Signed-off-by: Bibo Mao <maobibo@...ngson.cn>
> ---
> v1 ... v2:
> 1. Move guest_timing_exit_irqoff() after host interrupt handling like
> other architectures.
> 2. Construct interrupt context pt_regs from guest entering context
> 3. Add cond_resched() after irq enabling
> ---
> arch/loongarch/kernel/traps.c | 1 +
> arch/loongarch/kvm/vcpu.c | 36 ++++++++++++++++++++++++++++++++++-
> 2 files changed, 36 insertions(+), 1 deletion(-)
>
> diff --git a/arch/loongarch/kernel/traps.c b/arch/loongarch/kernel/traps.c
> index 2ec3106c0da3..eed0d8b02ee3 100644
> --- a/arch/loongarch/kernel/traps.c
> +++ b/arch/loongarch/kernel/traps.c
> @@ -1114,6 +1114,7 @@ asmlinkage void noinstr do_vint(struct pt_regs *regs, unsigned long sp)
>
> irqentry_exit(regs, state);
> }
> +EXPORT_SYMBOL(do_vint);
>
> unsigned long eentry;
> unsigned long tlbrentry;
> diff --git a/arch/loongarch/kvm/vcpu.c b/arch/loongarch/kvm/vcpu.c
> index 9e1a9b4aa4c6..bab7a71eb965 100644
> --- a/arch/loongarch/kvm/vcpu.c
> +++ b/arch/loongarch/kvm/vcpu.c
> @@ -5,6 +5,7 @@
>
> #include <linux/kvm_host.h>
> #include <linux/entry-kvm.h>
> +#include <asm/exception.h>
> #include <asm/fpu.h>
> #include <asm/lbt.h>
> #include <asm/loongarch.h>
> @@ -304,6 +305,23 @@ static int kvm_pre_enter_guest(struct kvm_vcpu *vcpu)
> return ret;
> }
>
> +static void kvm_handle_irq(struct kvm_vcpu *vcpu)
> +{
> + struct pt_regs regs, *old;
> +
> + /*
> + * Construct pseudo pt_regs, only necessary registers is added
> + * Interrupt context coming from guest enter context
> + */
> + old = (struct pt_regs *)(vcpu->arch.host_sp - sizeof(struct pt_regs));
> + /* Disable preemption in irq exit function irqentry_exit() */
> + regs.csr_prmd = 0;
> + regs.regs[LOONGARCH_GPR_SP] = vcpu->arch.host_sp;
> + regs.regs[LOONGARCH_GPR_FP] = old->regs[LOONGARCH_GPR_FP];
> + regs.csr_era = old->regs[LOONGARCH_GPR_RA];
> + do_vint(®s, (unsigned long)®s);
> +}
> +
> /*
> * Return 1 for resume guest and "<= 0" for resume host.
> */
> @@ -321,8 +339,23 @@ static int kvm_handle_exit(struct kvm_run *run, struct kvm_vcpu *vcpu)
>
> kvm_lose_pmu(vcpu);
>
> - guest_timing_exit_irqoff();
> guest_state_exit_irqoff();
> +
> + /*
> + * VM exit because of host interrupts
> + * Handle irq directly before enabling irq
> + */
> + if (!ecode && intr)
> + kvm_handle_irq(vcpu);
> +
> + /*
> + * Wait until after servicing IRQs to account guest time so that any
> + * ticks that occurred while running the guest are properly accounted
> + * to the guest. Waiting until IRQs are enabled degrades the accuracy
> + * of accounting via context tracking, but the loss of accuracy is
> + * acceptable for all known use cases.
> + */
> + guest_timing_exit_irqoff();
> local_irq_enable();
>
> trace_kvm_exit(vcpu, ecode);
> @@ -331,6 +364,7 @@ static int kvm_handle_exit(struct kvm_run *run, struct kvm_vcpu *vcpu)
> } else {
> WARN(!intr, "vm exiting with suspicious irq\n");
> ++vcpu->stat.int_exits;
> + cond_resched();
> }
>
> if (ret == RESUME_GUEST)
>
> base-commit: 80e54e84911a923c40d7bee33a34c1b4be148d7a
>
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