linux-kernel - Re: [PATCH] riscv: KVM: Remove unnecessary vcpu kick

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Message-ID: <20250220-1581569f8559049399549cae@orel>
Date: Thu, 20 Feb 2025 13:14:33 +0100
From: Andrew Jones <ajones@...tanamicro.com>
To: Radim Krčmář <rkrcmar@...tanamicro.com>
Cc: xiangwencheng <xiangwencheng@...xincomputing.com>, anup@...infault.org, 
	kvm-riscv@...ts.infradead.org, kvm@...r.kernel.org, linux-riscv@...ts.infradead.org, 
	linux-kernel@...r.kernel.org, atishp@...shpatra.org, paul.walmsley@...ive.com, 
	palmer@...belt.com, aou@...s.berkeley.edu, 
	linux-riscv <linux-riscv-bounces@...ts.infradead.org>
Subject: Re: [PATCH] riscv: KVM: Remove unnecessary vcpu kick

On Thu, Feb 20, 2025 at 09:50:06AM +0100, Radim Krčmář wrote:
> 2025-02-20T16:17:33+08:00, xiangwencheng <xiangwencheng@...xincomputing.com>:
> >> From: "Andrew Jones"<ajones@...tanamicro.com>
> >> On Thu, Feb 20, 2025 at 03:12:58PM +0800, xiangwencheng wrote:
> >> > In kvm_arch_vcpu_blocking it will enable guest external interrupt, which
> >
> >> > means wirting to VS_FILE will cause an interrupt. And the interrupt handler
> >
> >> > hgei_interrupt which is setted in aia_hgei_init will finally call kvm_vcpu_kick
> >
> >> > to wake up vCPU.
> 
> (Configure your mail client, so it doesn't add a newline between each
>  quoted line when replying.)
> 
> >> > So I still think is not necessary to call another kvm_vcpu_kick after writing to
> >> > VS_FILE.
> 
> So the kick wasn't there to mask some other bug, thanks.
> 
> >> Right, we don't need anything since hgei_interrupt() kicks for us, but if
> >> we do
> >> 
> >> @@ -973,8 +973,8 @@ int kvm_riscv_vcpu_aia_imsic_inject(struct kvm_vcpu *vcpu,
> >>         read_lock_irqsave(&imsic->vsfile_lock, flags);
> >> 
> >>         if (imsic->vsfile_cpu >= 0) {
> >> +               kvm_vcpu_wake_up(vcpu);
> >>                 writel(iid, imsic->vsfile_va + IMSIC_MMIO_SETIPNUM_LE);
> >> -               kvm_vcpu_kick(vcpu);
> >>         } else {
> >>                 eix = &imsic->swfile->eix[iid / BITS_PER_TYPE(u64)];
> >>                 set_bit(iid & (BITS_PER_TYPE(u64) - 1), eix->eip);
> >> 
> >> then we should be able to avoid taking a host interrupt.
> 
> The wakeup is asynchronous, and this would practically never avoid the
> host interrupt, but we'd do extra pointless work...
> I think it's much better just with the write.  (The wakeup would again
> make KVM look like it has a bug elsewhere.)

Ah yes, the wakeup is asynchronous. Just dropping the kick is the right
way to go then.

Thanks,
drew