2025-05-30T12:29:30-07:00, Atish Patra [off-list ref]:

On 5/30/25 4:09 AM, Radim Krčmář wrote:

quoted

2025-05-29T11:44:38-07:00, Atish Patra [off-list ref]:

quoted

On 5/29/25 3:24 AM, Radim Krčmář wrote:

quoted

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:

quoted

quoted

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.

(It is a gap to me. :])

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 says nothing about the implementation of said interface.

Returning 42 in x21 as a response to an ecall with 0x10 in a7 and 0x3 in
a6 is perfectly valid RISC-V implementation that KVM currently cannot
virtualize.

quoted

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 ?

This thread already has a lot of them, so to avoid repeating them, I
have to go into quite niche use-cases:
When developing M-mode software on RISC-V (when RISC-V has more useful
implementations than QEMU), a developer might want to accelerate the
S/U-modes in KVM.
It is also simpler to implement an old SBI interface (especially with
bugs/quirks) if virtualization just executes the old M-mode binary.

Why must KVM prevent userspace from virtualizing RISC-V?

I am assuming you are not hinting Nested virtualization which can be 
achieved with existing
ISA provided mechanisms and accelerated by SBI NACL.

Right, I am talking about virtualization of RISC-V, because I don't have
a crystal ball to figure out what users will want.