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

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Message-ID: <abedb067-b91f-8821-9bce-d27f6c4efdee@redhat.com>
Date:   Tue, 30 Jul 2019 13:26:38 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Anup Patel <Anup.Patel@....com>,
        Palmer Dabbelt <palmer@...ive.com>,
        Paul Walmsley <paul.walmsley@...ive.com>,
        Radim K <rkrcmar@...hat.com>
Cc:     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>,
        Anup Patel <anup@...infault.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 13/16] RISC-V: KVM: Add timer functionality

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

Paolo

> +				HRTIMER_MODE_ABS);
> +		t->is_set = true;
> +	} else
> +		kvm_riscv_vcpu_set_interrupt(vcpu, IRQ_S_TIMER);
> +