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

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Message-ID: <7fe9e845c33e49e4c215e12b1ee1b5ed86a95bc1.camel@wdc.com>
Date:   Wed, 31 Jul 2019 01:55:59 +0000
From:   Atish Patra <Atish.Patra@....com>
To:     "paul.walmsley@...ive.com" <paul.walmsley@...ive.com>,
        "rkrcmar@...hat.com" <rkrcmar@...hat.com>,
        "pbonzini@...hat.com" <pbonzini@...hat.com>,
        "palmer@...ive.com" <palmer@...ive.com>,
        Anup Patel <Anup.Patel@....com>
CC:     "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>,
        "anup@...infault.org" <anup@...infault.org>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>
Subject: Re: [RFC PATCH 13/16] RISC-V: KVM: Add timer functionality

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.

> guests will already pay a latency price due to the extra
> cost of the SBI call compared to a bare metal implementation. 

Yes. There are ongoing discussions to remove this SBI call completely. 
Hopefully, that will happen before any real hardware with
virtualization support shows up :).

>  Sooner or
> later you may want to implement something like x86's heuristic to
> advance the timer deadline by a few hundred nanoseconds; perhaps add
> a
> TODO now).
> 

I am not aware of this approach. I will take a look. Thanks.

Regards,
Atish
> Paolo
> 
> > +				HRTIMER_MODE_ABS);
> > +		t->is_set = true;
> > +	} else
> > +		kvm_riscv_vcpu_set_interrupt(vcpu, IRQ_S_TIMER);
> > +
> 
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> linux-riscv@...ts.infradead.org
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