linux-kernel - Re: [PATCH v3 9/9] RISC-V: KVM: Upgrade the supported SBI version to 3.0

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Message-Id: <DA9G60UI0ZLC.1KIWBXCTX0427@ventanamicro.com>
Date: Fri, 30 May 2025 13:09:43 +0200
From: Radim Krčmář <rkrcmar@...tanamicro.com>
To: "Atish Patra" <atish.patra@...ux.dev>, "Andrew Jones"
 <ajones@...tanamicro.com>
Cc: "Anup Patel" <anup@...infault.org>, "Will Deacon" <will@...nel.org>,
 "Mark Rutland" <mark.rutland@....com>, "Paul Walmsley"
 <paul.walmsley@...ive.com>, "Palmer Dabbelt" <palmer@...belt.com>,
 "Mayuresh Chitale" <mchitale@...tanamicro.com>,
 <linux-riscv@...ts.infradead.org>, <linux-arm-kernel@...ts.infradead.org>,
 <linux-kernel@...r.kernel.org>, <kvm@...r.kernel.org>,
 <kvm-riscv@...ts.infradead.org>, "linux-riscv"
 <linux-riscv-bounces@...ts.infradead.org>
Subject: Re: [PATCH v3 9/9] RISC-V: KVM: Upgrade the supported SBI version
 to 3.0

2025-05-29T11:44:38-07:00, Atish Patra <atish.patra@...ux.dev>:
> On 5/29/25 3:24 AM, Radim Krčmář wrote:
>> I originally gave up on the idea, but I feel kinda bad for Drew now, so
>> trying again:
>
> I am sorry if some of my replies came across in the wrong way. That was 
> never
> the intention.

I didn't mean to accuse you, my apologies.  I agree with Drew's
positions, so to expand on a question that wasn't touched in his mail:

>> Even if userspace wants SBI for the M-mode interface, security minded
> This is probably a 3rd one ? Why we want M-mode interface in the user 
> space ?

It is about turning KVM into an ISA accelerator.

A guest thinks it is running in S/HS-mode.
The ecall instruction traps to M-mode.  RISC-V H extension doesn't
accelerate M-mode, so we have to emulate the trap in software.

The ISA doesn't say that M-mode means SBI.  We try really hard to have
SBI on all RISC-V, but I think KVM is taking it a bit too far.

We can discuss how best to describe SBI, so userspace can choose to
accelerate the M-mode in KVM, but I think that the ability to emulate
M-mode in userspace should be provided.