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

From: Atish Patra <Atish.Patra@wdc.com>
To: "paul.walmsley@sifive.com" <paul.walmsley@sifive.com>,
	"rkrcmar@redhat.com" <rkrcmar@redhat.com>,
	"pbonzini@redhat.com" <pbonzini@redhat.com>,
	"palmer@sifive.com" <palmer@sifive.com>,
	Anup Patel <Anup.Patel@wdc.com>
Cc: Damien Le Moal <Damien.LeMoal@wdc.com>,
	"kvm@vger.kernel.org" <kvm@vger.kernel.org>,
	"anup@brainfault.org" <anup@brainfault.org>,
	"daniel.lezcano@linaro.org" <daniel.lezcano@linaro.org>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	"hch@infradead.org" <hch@infradead.org>,
	Alistair Francis <Alistair.Francis@wdc.com>,
	"linux-riscv@lists.infradead.org"
	<linux-riscv@lists.infradead.org>,
	"tglx@linutronix.de" <tglx@linutronix.de>
Subject: Re: [RFC PATCH 13/16] RISC-V: KVM: Add timer functionality
Date: Wed, 31 Jul 2019 01:55:59 +0000	[thread overview]
Message-ID: <7fe9e845c33e49e4c215e12b1ee1b5ed86a95bc1.camel@wdc.com> (raw)
In-Reply-To: <abedb067-b91f-8821-9bce-d27f6c4efdee@redhat.com>

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);
> > +
> 
> _______________________________________________
> linux-riscv mailing list
> linux-riscv@lists.infradead.org
> http://lists.infradead.org/mailman/listinfo/linux-riscv

_______________________________________________
linux-riscv mailing list
linux-riscv@lists.infradead.org
http://lists.infradead.org/mailman/listinfo/linux-riscv