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

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Message-ID: <CAJF2gTS6FKdXhG3d944B+k8YLRw4JKLEnUtgeC2hDLUjLkhjbg@mail.gmail.com>
Date: Fri, 17 Oct 2025 17:53:50 +0800
From: Guo Ren <guoren@...nel.org>
To: Anup Patel <anup@...infault.org>
Cc: fangyu.yu@...ux.alibaba.com, atish.patra@...ux.dev, pjw@...nel.org, 
	palmer@...belt.com, aou@...s.berkeley.edu, alex@...ti.fr, 
	liujingqi@...xincomputing.com, 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 Fri, Oct 17, 2025 at 2:59 PM Anup Patel <anup@...infault.org> wrote:
>
> 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
Thx Anup, then Fangyu needn't update v2.

This patch also reveals the idea of our CSR_HGEIP design - Per-CPU access.
Although arm64 & x86 don't have a similar problem by using a shared
memory address map, it would be a complex hw design. The RISC-V
CSR_HGEIP design is much simpler for hw, and vCPU migration on the
pCPUs could tolerate one additional VM_enter/exit cost for an idle
vcpu.

>
> 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
> >



-- 
Best Regards
 Guo Ren