linux-kernel - Re: [PATCH 3/5] KVM: RISC-V: remove unnecessary SBI reset state

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Message-ID: <CAK9=C2W3gLpeOmOcX3sp+J1zuBkWek7nO8tV79=gaWx+QZ6rgQ@mail.gmail.com>
Date: Tue, 29 Apr 2025 11:20:00 +0530
From: Anup Patel <apatel@...tanamicro.com>
To: Radim Krčmář <rkrcmar@...tanamicro.com>
Cc: Anup Patel <anup@...infault.org>, kvm-riscv@...ts.infradead.org, kvm@...r.kernel.org, 
	linux-riscv@...ts.infradead.org, linux-kernel@...r.kernel.org, 
	Atish Patra <atishp@...shpatra.org>, Paul Walmsley <paul.walmsley@...ive.com>, 
	Palmer Dabbelt <palmer@...belt.com>, Albert Ou <aou@...s.berkeley.edu>, 
	Alexandre Ghiti <alex@...ti.fr>, Andrew Jones <ajones@...tanamicro.com>, 
	Mayuresh Chitale <mchitale@...tanamicro.com>
Subject: Re: [PATCH 3/5] KVM: RISC-V: remove unnecessary SBI reset state

On Mon, Apr 28, 2025 at 11:31 PM Radim Krčmář <rkrcmar@...tanamicro.com> wrote:
>
> 2025-04-28T17:46:01+05:30, Anup Patel <anup@...infault.org>:
> > On Thu, Apr 3, 2025 at 5:02 PM Radim Krčmář <rkrcmar@...tanamicro.com> wrote:
> >>
> >> The SBI reset state has only two variables -- pc and a1.
> >> The rest is known, so keep only the necessary information.
> >>
> >> The reset structures make sense if we want userspace to control the
> >> reset state (which we do), but I'd still remove them now and reintroduce
> >> with the userspace interface later -- we could probably have just a
> >> single reset state per VM, instead of a reset state for each VCPU.
> >
> > The SBI spec does not define the reset state of CPUs. The SBI
> > implementations (aka KVM RISC-V or OpenSBI) or platform
> > firmwares are free to clear additional registers as part system
> > reset or CPU.
> >
> > As part of resetting the VCPU, the in-kernel KVM clears all
> > the registers.
>
> Yes, but instead of doing a simple memset(0), KVM carriers around a lot
> of data with minimal information value.  Reset is not really a fast
> path, so I think it would be good to have the code there as simple as
> possible.
>
> > The setting of PC, A0, and A1 is only an entry condition defined
> > for CPUs brought-up using SBI HSM start or SBI System suspend.
>
> That is why this patch has to add kvm_vcpu_reset_state, to remember the
> state of pc and a1.  (a0 is hart id and can be figured out.)
>
> > We should not go ahead with this patch.
>
> This patch only does refactoring.  Do you think the current reset
> structures are better?
>

I am fine getting rid of reset structures as long as we have
memset(0) in-place to achieve the same thing.

Regards,
Anup