Re: [RFC PATCH 05/16] RISC-V: KVM: Implement VCPU interrupts and requests handling

[Anup Patel <anup@brainfault.org>]

On Tue, Jul 30, 2019 at 7:38 PM Paolo Bonzini <pbonzini@redhat.com> wrote:
>
> On 30/07/19 15:35, Anup Patel wrote:
> > On Tue, Jul 30, 2019 at 6:48 PM Paolo Bonzini <pbonzini@redhat.com> wrote:
> >>
> >> On 30/07/19 14:45, Anup Patel wrote:
> >>> Here's some text from RISC-V spec regarding SIP CSR:
> >>> "software interrupt-pending (SSIP) bit in the sip register. A pending
> >>> supervisor-level software interrupt can be cleared by writing 0 to the SSIP bit
> >>> in sip. Supervisor-level software interrupts are disabled when the SSIE bit in
> >>> the sie register is clear."
> >>>
> >>> Without RISC-V hypervisor extension, the SIP is essentially a restricted
> >>> view of MIP CSR. Also as-per above, S-mode SW can only write 0 to SSIP
> >>> bit in SIP CSR whereas it can only be set by M-mode SW or some HW
> >>> mechanism (such as S-mode CLINT).
> >>
> >> But that's not what the spec says.  It just says (just before the
> >> sentence you quoted):
> >>
> >>    A supervisor-level software interrupt is triggered on the current
> >>    hart by writing 1 to its supervisor software interrupt-pending (SSIP)
> >>    bit in the sip register.
> >
> > Unfortunately, this statement does not state who is allowed to write 1
> > in SIP.SSIP bit.
>
> If it doesn't state who is allowed to write 1, whoever has access to sip
> can.
>
> > I quoted MIP CSR documentation to highlight the fact that only M-mode
> > SW can set SSIP bit.
> >
> > In fact, I had same understanding as you have regarding SSIP bit
> > until we had MSIP issue in OpenSBI.
> > (https://github.com/riscv/opensbi/issues/128)
> >
> >> and it's not written anywhere that S-mode SW cannot write 1.  In fact
> >> that text is even under sip, not under mip, so IMO there's no doubt that
> >> S-mode SW _can_ write 1, and the hypervisor must operate accordingly.
> >
> > Without hypervisor support, SIP CSR is nothing but a restricted view of
> > MIP CSR thats why MIP CSR documentation applies here.
>
> But the privileged spec says mip.MSIP is read-only, it cannot be cleared
> (as in the above OpenSBI issue).  So mip.MSIP and sip.SSIP are already
> different in that respect, and I don't see how the spec says that S-mode
> SW cannot set sip.SSIP.
>
> (As an aside, why would M-mode even bother using sip and not mip to
> write 1 to SSIP?).
>
> > I think this discussion deserves a Github issue on RISC-V ISA manual.
>
> Perhaps, but I think it makes more sense this way.  The question remains
> of why M-mode is not allowed to write to MSIP/MEIP/MTIP.  My guess is
> that then MSIP/MEIP/MTIP are simply a read-only view of an external pin,
> so it simplifies hardware a tiny bit by forcing acks to go through the
> MMIO registers.
>
> > If my interpretation is incorrect then it would be really strange that
> > HART in S-mode SW can inject IPI to itself by writing 1 to SIP.SSIP bit.
>
> Well, it can be useful, for example Windows does it when interrupt
> handlers want to schedule some work to happen out of interrupt context.
>  Going through SBI would be unpleasant if it causes an HS-mode trap.

Another way of artificially injecting interrupt would be using interrupt
controller, where Windows can just write to some pending register of
interrupt controller.

I have raised a new Github issue on GitHub for clarity on this. You can
add your comments to this issue as well.
https://github.com/riscv/riscv-isa-manual/issues/425

Also, I have raised a proposal to support mechanism for external entity
(such as PLICv2 with virtualization support) to inject virtual interrupts.
https://github.com/riscv/riscv-isa-manual/issues/429

Regards,
Anup

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