[PATCH 4/5] KVM: RISC-V: reset VCPU state when becoming runnable

[Anup Patel]

On Tue, Apr 29, 2025 at 3:55 PM Radim Krčmář <rkrcmar at ventanamicro.com> wrote:
>
> 2025-04-29T11:25:35+05:30, Anup Patel <apatel at ventanamicro.com>:
> > On Mon, Apr 28, 2025 at 11:15 PM Radim Krčmář <rkrcmar at ventanamicro.com> wrote:
> >>
> >> 2025-04-28T17:52:25+05:30, Anup Patel <anup at brainfault.org>:
> >> > On Thu, Apr 3, 2025 at 5:02 PM Radim Krčmář <rkrcmar at ventanamicro.com> wrote:
> >> >> For a cleaner solution, we should add interfaces to perform the KVM-SBI
> >> >> reset request on userspace demand.  I think it would also be much better
> >> >> if userspace was in control of the post-reset state.
> >> >
> >> > Apart from breaking KVM user-space, this patch is incorrect and
> >> > does not align with the:
> >> > 1) SBI spec
> >> > 2) OS boot protocol.
> >> >
> >> > The SBI spec only defines the entry state of certain CPU registers
> >> > (namely, PC, A0, and A1) when CPU enters S-mode:
> >> > 1) Upon SBI HSM start call from some other CPU
> >> > 2) Upon resuming from non-retentive SBI HSM suspend or
> >> >     SBI system suspend
> >> >
> >> > The S-mode entry state of the boot CPU is defined by the
> >> > OS boot protocol and not by the SBI spec. Due to this, reason
> >> > KVM RISC-V expects user-space to set up the S-mode entry
> >> > state of the boot CPU upon system reset.
> >>
> >> We can handle the initial state consistency in other patches.
> >> What needs addressing is a way to trigger the KVM reset from userspace,
> >> even if only to clear the internal KVM state.
> >>
> >> I think mp_state is currently the best signalization that KVM should
> >> reset, so I added it there.
> >>
> >> What would be your preferred interface for that?
> >>
> >
> > Instead of creating a new interface, I would prefer that VCPU
> > which initiates SBI System Reset should be resetted immediately
> > in-kernel space before forwarding the system reset request to
> > user space.
>
> The initiating VCPU might not be the boot VCPU.
> It would be safer to reset all of them.

I meant initiating VCPU and not the boot VCPU. Currently, the
non-initiating VCPUs are already resetted by VCPU requests
so nothing special needs to be done.

>
> You also previously mentioned that we need to preserve the pre-reset
> state for userspace, which I completely agree with and it is why the
> reset happens later.

Yes, that was only for debug purposes from user space. At the
moment, there is no one using this for debug purposes so we
can sacrifice that.

>
> >             This way we also force KVM user-space to explicitly
> > set the PC, A0, and A1 before running the VCPU again after
> > system reset.
>
> We also want to consider reset from emulation outside of KVM.
>
> There is a "simple" solution that covers everything (except speed) --
> the userspace can tear down the whole VM and re-create it.
> Do we want to do this instead and drop all resets from KVM?

I think we should keep the VCPU resets in KVM so that handling
of system reset handling in user space remains simple. The user
space can also re-create the VM upon system reset but that is
user space choice.

Regards,
Anup