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

["Radim Krčmář" <rkrcmar@ventanamicro.com>]

2025-05-29T11:44:38-07:00, Atish Patra <atish.patra@linux.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.