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

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Message-ID: <0dcd01cd-419f-4225-b22c-cbaf82718235@linux.dev>
Date: Fri, 30 May 2025 12:29:30 -0700
From: Atish Patra <atish.patra@...ux.dev>
To: Radim Krčmář <rkrcmar@...tanamicro.com>,
 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


On 5/30/25 4:09 AM, Radim Krčmář wrote:
> 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.
We don't need to accelerate M-mode. That's the beauty of the RISC-V H 
extension.
The ISA is designed in such a way that the SBI is the interface between 
the supervisor environment (VS/HS)
and the supervisor execution environment (HS/M).


>
> 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.
I am still trying to understand the advantages of emulating the M-mode 
in the user space.
Can you please elaborate ?
I am assuming you are not hinting Nested virtualization which can be 
achieved with existing
ISA provided mechanisms and accelerated by SBI NACL.