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

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Message-Id: <D9IGVF0OY4WJ.1O1BX0M2LWUVM@ventanamicro.com>
Date: Mon, 28 Apr 2025 20:00:54 +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 3/5] KVM: RISC-V: remove unnecessary SBI reset state

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?

Thanks.