From: Marc Zyngier <maz@kernel.org>
To: Jianyong Wu <Jianyong.Wu@arm.com>
Cc: netdev@vger.kernel.org, yangbo.lu@nxp.com,
john.stultz@linaro.org, tglx@linutronix.de, pbonzini@redhat.com,
sean.j.christopherson@intel.com, richardcochran@gmail.com,
Mark Rutland <Mark.Rutland@arm.com>,
will@kernel.org, Suzuki Poulose <Suzuki.Poulose@arm.com>,
Steven Price <Steven.Price@arm.com>,
linux-kernel@vger.kernel.org,
linux-arm-kernel@lists.infradead.org,
kvmarm@lists.cs.columbia.edu, kvm@vger.kernel.org,
Steve Capper <Steve.Capper@arm.com>, Kaly Xin <Kaly.Xin@arm.com>,
Justin He <Justin.He@arm.com>, nd <nd@arm.com>
Subject: Re: [RFC PATCH v9 7/8] ptp: arm64: Enable ptp_kvm for arm64
Date: Fri, 10 Jan 2020 10:35:02 +0000 [thread overview]
Message-ID: <a85deebc23c1fa77e6f70b6eaef22a34@kernel.org> (raw)
In-Reply-To: <HE1PR0801MB16765B52E5DCD8EA480EDABFF4380@HE1PR0801MB1676.eurprd08.prod.outlook.com>
Hi Jianyong,
On 2020-01-10 10:15, Jianyong Wu wrote:
> Hi Marc,
[...]
>> >> > + ktime_overall = hvc_res.a0 << 32 | hvc_res.a1;
>> >> > + *ts = ktime_to_timespec64(ktime_overall);
>> >> > + *cycle = hvc_res.a2 << 32 | hvc_res.a3;
>> >>
>> >> So why isn't that just a read of the virtual counter, given that what
>> >> you do in the hypervisor seems to be "cntpct - cntvoff"?
>> >>
>> >> What am I missing here?
>> >>
>> > We need get clock time and counter cycle at the same time, so we can't
>> > just read virtual counter at guest and must get it from host.
>>
>> See my comment in my reply to patch #6: *Must* seems like a very
>> strong
>> word, and you don't explain *why* that's better than just computing
>> the
>> total hypercall cost. Hint: given the frequency of the counter (in the
>> few MHz
>> range) vs the frequency of a CPU (in the multiple GHz range, and with
>> an IPC
>> close enough to 1), I doubt that you'll see the counter making much
>> progress
>> across a hypercall.
>>
> Sorry, I will avoid to use those strong words.
>
> It's really the case that the hypercall won't across cycle in general.
> But sometimes, kernel preempt
> may happen in the middle of the hypercall which we can't assume how
> long before schedule back. so it's better capture them
> together at the same time.
Fair enough. Please document the rational, as I guess others will ask
the same questions.
Then the problem to solve is that of the reference counter, as you so
far
assume the virtual counter. I guess you need to be able to let the guest
select the reference counter when calling the PTP service.
[...]
> By the way, does nested virtualization diff between arm64 and arm32?
There is no nested virt for 32bit (it is explicitly forbidden by the
architecture).
M.
--
Jazz is not dead. It just smells funny...
next prev parent reply other threads:[~2020-01-10 10:35 UTC|newest]
Thread overview: 30+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-12-10 3:40 [RFC PATCH v9 0/8] Enable ptp_kvm for arm64 Jianyong Wu
2019-12-10 3:40 ` [RFC PATCH v9 1/8] arm/arm64: smccc/psci: add arm_smccc_1_1_get_conduit() Jianyong Wu
2019-12-10 3:40 ` [RFC PATCH v9 2/8] psci: let arm_smccc_1_1_invoke available by modules Jianyong Wu
2019-12-10 3:40 ` [RFC PATCH v9 3/8] ptp: Reorganize ptp_kvm modules to make it arch-independent Jianyong Wu
2019-12-10 3:40 ` [RFC PATCH v9 4/8] time: Add mechanism to recognize clocksource in time_get_snapshot Jianyong Wu
2019-12-10 3:40 ` [RFC PATCH v9 5/8] clocksource: Add clocksource id for arm arch counter Jianyong Wu
2019-12-10 3:40 ` [RFC PATCH v9 6/8] psci: Add hvc call service for ptp_kvm Jianyong Wu
2020-01-07 9:16 ` Marc Zyngier
2020-01-09 5:45 ` Jianyong Wu
2020-01-09 9:16 ` Marc Zyngier
2020-01-10 9:51 ` Jianyong Wu
2020-01-10 10:56 ` Marc Zyngier
2020-01-13 10:30 ` Jianyong Wu
2020-01-13 11:16 ` Marc Zyngier
2020-01-14 10:34 ` Jianyong Wu
2019-12-10 3:40 ` [RFC PATCH v9 7/8] ptp: arm64: Enable ptp_kvm for arm64 Jianyong Wu
2020-01-07 9:29 ` Marc Zyngier
2020-01-09 5:59 ` Jianyong Wu
2020-01-09 9:24 ` Marc Zyngier
2020-01-09 9:46 ` Marc Zyngier
2020-01-10 10:15 ` Jianyong Wu
2020-01-10 10:15 ` Jianyong Wu
2020-01-10 10:35 ` Marc Zyngier [this message]
2020-01-13 10:37 ` Jianyong Wu
2020-01-13 11:21 ` Marc Zyngier
2020-01-14 10:22 ` Jianyong Wu
2019-12-10 3:40 ` [RFC PATCH v9 8/8] kvm: arm64: Add capability check extension for ptp_kvm Jianyong Wu
2020-01-06 9:38 ` [RFC PATCH v9 0/8] Enable ptp_kvm for arm64 Jianyong Wu
2020-01-07 8:15 ` Paolo Bonzini
2020-01-07 9:33 ` Marc Zyngier
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