linux-kernel - Re: [PATCH v3 2/2] RISC-V KVM: Remove unnecessary HGATP csr_read

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Message-ID: <CAJF2gTRR0LTO-CbUMkyXisov3BeftN+UJzv99GX7KfqCxWxmUg@mail.gmail.com>
Date: Thu, 21 Aug 2025 09:25:58 +0800
From: Guo Ren <guoren@...nel.org>
To: Troy Mitchell <troy.mitchell@...ux.dev>
Cc: fangyu.yu@...ux.alibaba.com, anup@...infault.org, atish.patra@...ux.dev, 
	paul.walmsley@...ive.com, palmer@...belt.com, aou@...s.berkeley.edu, 
	alex@...ti.fr, kvm@...r.kernel.org, kvm-riscv@...ts.infradead.org, 
	linux-riscv@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 2/2] RISC-V KVM: Remove unnecessary HGATP csr_read

On Wed, Aug 20, 2025 at 10:40 PM Troy Mitchell <troy.mitchell@...ux.dev> wrote:
>
> On Wed, Aug 20, 2025 at 08:59:52PM +0800, fangyu.yu@...ux.alibaba.com wrote:
> > From: "Guo Ren (Alibaba DAMO Academy)" <guoren@...nel.org>
> >
> > The HGATP has been set to zero in gstage_mode_detect(), so there
> > is no need to save the old context. Unify the code convention
> > with gstage_mode_detect().
> >
> > Signed-off-by: Guo Ren (Alibaba DAMO Academy) <guoren@...nel.org>
> > Signed-off-by: Fangyu Yu <fangyu.yu@...ux.alibaba.com>
> > ---
> >  arch/riscv/kvm/vmid.c | 5 +----
> >  1 file changed, 1 insertion(+), 4 deletions(-)
> >
> > diff --git a/arch/riscv/kvm/vmid.c b/arch/riscv/kvm/vmid.c
> > index 5f33625f4070..abb1c2bf2542 100644
> > --- a/arch/riscv/kvm/vmid.c
> > +++ b/arch/riscv/kvm/vmid.c
> > @@ -25,15 +25,12 @@ static DEFINE_SPINLOCK(vmid_lock);
> >
> >  void __init kvm_riscv_gstage_vmid_detect(void)
> >  {
> > -     unsigned long old;
> > -
> >       /* Figure-out number of VMID bits in HW */
> > -     old = csr_read(CSR_HGATP);
> >       csr_write(CSR_HGATP, (kvm_riscv_gstage_mode << HGATP_MODE_SHIFT) | HGATP_VMID);
> >       vmid_bits = csr_read(CSR_HGATP);
> >       vmid_bits = (vmid_bits & HGATP_VMID) >> HGATP_VMID_SHIFT;
> >       vmid_bits = fls_long(vmid_bits);
> > -     csr_write(CSR_HGATP, old);
> > +     csr_write(CSR_HGATP, 0);
> Is setting HGATP to 0 in gstage_mode_detect meaningless now?
> If so, it might be better to drop it and just keep the one here.
Sorry, I misunderstood here.

1. kvm_riscv_gstage_vmid_detect() & gstage_mode_detect() are indepent
function, so keep csr_write(CSR_HGATP, 0) is considerable.

2. But your idea is good, because csr_write(CSR_HGATP, 0) would cause
TLB flush in some micro-arch, which reduces the IPC. So, removing
unnecessary CSR_write (CSR_HGATP, 0) is also considerable.

I would update V4 to merge kvm_riscv_gstage_vmid_detect() &
gstage_mode_detect() into a single function, inspired by your idea.

>
>                 - Troy
>
> >
> >       /* We polluted local TLB so flush all guest TLB */
> >       kvm_riscv_local_hfence_gvma_all();
> > --
> > 2.49.0
> >
> >
> > _______________________________________________
> > linux-riscv mailing list
> > linux-riscv@...ts.infradead.org
> > http://lists.infradead.org/mailman/listinfo/linux-riscv

-- 
Best Regards
 Guo Ren

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