linux-kernel - Re: [PATCH] RISC-V: KVM: Disable instret/cycle for VU mode by default

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Message-ID: <CAAhSdy2DG0y3r8T=AqJ-T7+VaVcVpY0pSRZDptfYwwknshB+zg@mail.gmail.com>
Date: Thu, 15 May 2025 11:10:34 +0530
From: Anup Patel <anup@...infault.org>
To: Atish Kumar Patra <atishp@...osinc.com>
Cc: Atish Patra <atishp@...shpatra.org>, Paul Walmsley <paul.walmsley@...ive.com>, 
	Palmer Dabbelt <palmer@...belt.com>, Alexandre Ghiti <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] RISC-V: KVM: Disable instret/cycle for VU mode by default

On Thu, May 15, 2025 at 2:31 AM Atish Kumar Patra <atishp@...osinc.com> wrote:
>
> On Wed, May 14, 2025 at 3:55 AM Anup Patel <anup@...infault.org> wrote:
> >
> > On Wed, May 14, 2025 at 12:13 PM Atish Patra <atishp@...osinc.com> wrote:
> > >
> > > The KVM virtualizes PMU in RISC-V and disables all counter access except
> > > TM bit by default vi hstateen CSR. There is no benefit in enabling CY/TM
> > > bits in scounteren for the guest user space as it can't be run without
> > > hcounteren anyways.
> > >
> > > Allow only TM bit which matches the hcounteren default setting.
> > >
> > > Signed-off-by: Atish Patra <atishp@...osinc.com>
> > > ---
> > >  arch/riscv/kvm/vcpu.c | 4 ++--
> > >  1 file changed, 2 insertions(+), 2 deletions(-)
> > >
> > > diff --git a/arch/riscv/kvm/vcpu.c b/arch/riscv/kvm/vcpu.c
> > > index 60d684c76c58..873593bfe610 100644
> > > --- a/arch/riscv/kvm/vcpu.c
> > > +++ b/arch/riscv/kvm/vcpu.c
> > > @@ -146,8 +146,8 @@ int kvm_arch_vcpu_create(struct kvm_vcpu *vcpu)
> > >         if (kvm_riscv_vcpu_alloc_vector_context(vcpu, cntx))
> > >                 return -ENOMEM;
> > >
> > > -       /* By default, make CY, TM, and IR counters accessible in VU mode */
> > > -       reset_csr->scounteren = 0x7;
> > > +       /* By default, only TM should be accessible in VU mode */
> > > +       reset_csr->scounteren = 0x2;
> >
> > Let's remove this as well because the Linux SBI PMU driver
> > does initialize scounteren correctly.
> >
>
> But other guests may not. I thought time should be a basic one that
> should be allowed by default.
>

There is no specification (SBI or Priv spec) which mandates M-mode
or HS-mode to setup S-mode CSRs. Setting scounteren bits (including
TM bit) has always been a HACK or work-around.

It is better to remove scounteren initialization HACK from KVM RISC-V
before more supervisor software starts depending on it.

Regards,
Anup