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

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Date:   Wed, 31 Jul 2019 08:58:09 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     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>
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 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.

Paolo