Messages in this thread | | | Date | Mon, 13 Jan 2020 11:21:11 +0000 | From | Marc Zyngier <> | Subject | Re: [RFC PATCH v9 7/8] ptp: arm64: Enable ptp_kvm for arm64 |
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On 2020-01-13 10:37, Jianyong Wu wrote: > Hi Marc, > >> -----Original Message----- >> From: Marc Zyngier <maz@kernel.org> >> Sent: Friday, January 10, 2020 6:35 PM >> 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 >> >> 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. >> > Ok > >> 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. >> > I could not come up with an idea about the point where the guest give > this info of counter value. > Where we give that interface to ptp service, as it's not a user space > application.
Again: why don't you let the guest ask for the counter it wants as part of the SMC call? What is preventing this?
M. -- Jazz is not dead. It just smells funny...
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