linux-kernel - Re: [RFC PATCH 13/16] RISC-V: KVM: Add timer functionality

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Date:   Wed, 31 Jul 2019 12:48:40 +0530
From:   Anup Patel <anup@...infault.org>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     Atish Patra <Atish.Patra@....com>,
        "paul.walmsley@...ive.com" <paul.walmsley@...ive.com>,
        "rkrcmar@...hat.com" <rkrcmar@...hat.com>,
        "palmer@...ive.com" <palmer@...ive.com>,
        Anup Patel <Anup.Patel@....com>,
        "linux-riscv@...ts.infradead.org" <linux-riscv@...ts.infradead.org>,
        "daniel.lezcano@...aro.org" <daniel.lezcano@...aro.org>,
        Alistair Francis <Alistair.Francis@....com>,
        Damien Le Moal <Damien.LeMoal@....com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "hch@...radead.org" <hch@...radead.org>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>
Subject: Re: [RFC PATCH 13/16] RISC-V: KVM: Add timer functionality

On Wed, Jul 31, 2019 at 12:28 PM Paolo Bonzini <pbonzini@...hat.com> wrote:
>
> On 31/07/19 03:55, Atish Patra wrote:
> > On Tue, 2019-07-30 at 13:26 +0200, Paolo Bonzini wrote:
> >> On 29/07/19 13:57, Anup Patel wrote:
> >>> +   if (delta_ns > VCPU_TIMER_PROGRAM_THRESHOLD_NS) {
> >>> +           hrtimer_start(&t->hrt, ktime_add_ns(ktime_get(),
> >>> delta_ns),
> >>
> >> I think the guest would prefer if you saved the time before enabling
> >> interrupts on the host, and use that here instead of ktime_get().
> >> Otherwise the timer could be delayed arbitrarily by host interrupts.
> >>
> >> (Because the RISC-V SBI timer is relative only---which is
> >> unfortunate---
> >
> > Just to clarify: RISC-V SBI timer call passes absolute time.
> >
> > https://elixir.bootlin.com/linux/v5.3-rc2/source/drivers/clocksource/timer-riscv.c#L32
> >
> > That's why we compute a delta between absolute time passed via SBI and
> > current time. hrtimer is programmed to trigger only after the delta
> > time from now.
>
> Nevermind, I got lost in all the conversions.
>
> One important issue is the lack of ability to program a delta between
> HS/HU-mode cycles and VS/VU-mode cycles.  Without this, it's impossible
> to do virtual machine migration (except with hcounteren
> trap-and-emulate, which I think we agree is not acceptable).  I found
> the open issue at https://github.com/riscv/riscv-isa-manual/issues/298
> and commented on it.

This Github issue is open since quite some time now.

Thanks for commenting. I have pinged RISC-V spec maintainers as well.

Regards,
Anup