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

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Message-ID: <CAAhSdy19_dEL7e_sEFYi-hXvhVerm_cr3BdZ-TRw0aTTL-O9ZQ@mail.gmail.com>
Date:   Tue, 30 Jul 2019 19:05:40 +0530
From:   Anup Patel <anup@...infault.org>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     Anup Patel <Anup.Patel@....com>,
        Palmer Dabbelt <palmer@...ive.com>,
        Paul Walmsley <paul.walmsley@...ive.com>,
        Radim K <rkrcmar@...hat.com>,
        Daniel Lezcano <daniel.lezcano@...aro.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Atish Patra <Atish.Patra@....com>,
        Alistair Francis <Alistair.Francis@....com>,
        Damien Le Moal <Damien.LeMoal@....com>,
        Christoph Hellwig <hch@...radead.org>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "linux-riscv@...ts.infradead.org" <linux-riscv@...ts.infradead.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [RFC PATCH 05/16] RISC-V: KVM: Implement VCPU interrupts and
 requests handling

On Tue, Jul 30, 2019 at 6:48 PM Paolo Bonzini <pbonzini@...hat.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.

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.

I think this discussion deserves a Github issue on RISC-V ISA manual.

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.

>
> In fact I'm sure that if Windows were ever ported to RISC-V, it would be
> very happy to use that feature.  On x86, Intel even accelerated it
> specifically for Microsoft. :)

That would be indeed very strange usage.  :)

Regards,
Anup