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

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Message-Id: <D9IGJR9DGFAM.1PVHVOOTVRFZW@ventanamicro.com>
Date: Mon, 28 Apr 2025 19:45:41 +0200
From: Radim Krčmář <rkrcmar@...tanamicro.com>
To: "Anup Patel" <anup@...infault.org>
Cc: <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

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?

Thanks.