linux-kernel - Re: [PATCH 4/5] KVM: RISC-V: reset VCPU state when becoming runnable

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Message-ID: <CAK9=C2Woc5MtrJeqNtaVkMXWEsGeZPsmUgtFQET=OKLHLwRbPA@mail.gmail.com>
Date: Tue, 29 Apr 2025 11:25:35 +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 4/5] KVM: RISC-V: reset VCPU state when becoming runnable

On Mon, Apr 28, 2025 at 11:15 PM Radim Krčmář <rkrcmar@...tanamicro.com> wrote:
>
> 2025-04-28T17:52:25+05:30, Anup Patel <anup@...infault.org>:
> > On Thu, Apr 3, 2025 at 5:02 PM Radim Krčmář <rkrcmar@...tanamicro.com> wrote:
> >> For a cleaner solution, we should add interfaces to perform the KVM-SBI
> >> reset request on userspace demand.  I think it would also be much better
> >> if userspace was in control of the post-reset state.
> >
> > Apart from breaking KVM user-space, this patch is incorrect and
> > does not align with the:
> > 1) SBI spec
> > 2) OS boot protocol.
> >
> > The SBI spec only defines the entry state of certain CPU registers
> > (namely, PC, A0, and A1) when CPU enters S-mode:
> > 1) Upon SBI HSM start call from some other CPU
> > 2) Upon resuming from non-retentive SBI HSM suspend or
> >     SBI system suspend
> >
> > The S-mode entry state of the boot CPU is defined by the
> > OS boot protocol and not by the SBI spec. Due to this, reason
> > KVM RISC-V expects user-space to set up the S-mode entry
> > state of the boot CPU upon system reset.
>
> We can handle the initial state consistency in other patches.
> What needs addressing is a way to trigger the KVM reset from userspace,
> even if only to clear the internal KVM state.
>
> I think mp_state is currently the best signalization that KVM should
> reset, so I added it there.
>
> What would be your preferred interface for that?
>

Instead of creating a new interface, I would prefer that VCPU
which initiates SBI System Reset should be resetted immediately
in-kernel space before forwarding the system reset request to
user space. This way we also force KVM user-space to explicitly
set the PC, A0, and A1 before running the VCPU again after
system reset.

Regards,
Anup