linux-kernel - Re: [PATCH] RISC-V: KVM: Read HGEIP CSR on the correct cpu

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Message-ID: <CAAhSdy2UcmoPLF0CGBrsF1bRdJe-X05YA7UQOVffxBjZTourMA@mail.gmail.com>
Date: Fri, 17 Oct 2025 12:28:44 +0530
From: Anup Patel <anup@...infault.org>
To: fangyu.yu@...ux.alibaba.com
Cc: atish.patra@...ux.dev, pjw@...nel.org, palmer@...belt.com, 
	aou@...s.berkeley.edu, alex@...ti.fr, liujingqi@...xincomputing.com, 
	guoren@...nel.org, kvm@...r.kernel.org, kvm-riscv@...ts.infradead.org, 
	linux-riscv@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] RISC-V: KVM: Read HGEIP CSR on the correct cpu

On Thu, Oct 16, 2025 at 6:57 AM <fangyu.yu@...ux.alibaba.com> wrote:
>
> From: Fangyu Yu <fangyu.yu@...ux.alibaba.com>
>
> When executing kvm_riscv_vcpu_aia_has_interrupts, the vCPU may have
> migrated and the IMSIC VS-file have not been updated yet, currently
> the HGEIP CSR should be read from the imsic->vsfile_cpu ( the pCPU
> before migration ) via on_each_cpu_mask, but this will trigger an
> IPI call and repeated IPI within a period of time is expensive in
> a many-core systems.
>
> Just let the vCPU execute and update the correct IMSIC VS-file via
> kvm_riscv_vcpu_aia_imsic_update may be a simple solution.
>
> Fixes: 4cec89db80ba ("RISC-V: KVM: Move HGEI[E|P] CSR access to IMSIC virtualization")
> Signed-off-by: Fangyu Yu <fangyu.yu@...ux.alibaba.com>
> ---
>  arch/riscv/kvm/aia_imsic.c | 8 ++++++--
>  1 file changed, 6 insertions(+), 2 deletions(-)
>
> diff --git a/arch/riscv/kvm/aia_imsic.c b/arch/riscv/kvm/aia_imsic.c
> index fda0346f0ea1..168c02ad0a78 100644
> --- a/arch/riscv/kvm/aia_imsic.c
> +++ b/arch/riscv/kvm/aia_imsic.c
> @@ -689,8 +689,12 @@ bool kvm_riscv_vcpu_aia_imsic_has_interrupt(struct kvm_vcpu *vcpu)
>          */
>
>         read_lock_irqsave(&imsic->vsfile_lock, flags);
> -       if (imsic->vsfile_cpu > -1)
> -               ret = !!(csr_read(CSR_HGEIP) & BIT(imsic->vsfile_hgei));
> +       if (imsic->vsfile_cpu > -1) {
> +               if (imsic->vsfile_cpu != smp_processor_id())

Good catch !!!

I agree with Guo Ren. We should use "vcpu->cpu" over here
instead of smp_processor_id(). Also, I think we should add
some comments for future reference. I will take care of this
at the time of merging this patch.

Queued this as fixes for Linux-6.18

Thanks,
Anup

> +                       ret = true;
> +               else
> +                       ret = !!(csr_read(CSR_HGEIP) & BIT(imsic->vsfile_hgei));
> +       }
>         read_unlock_irqrestore(&imsic->vsfile_lock, flags);
>
>         return ret;
> --
> 2.50.1
>