Hi Marc, > -----Original Message----- > From: Marc Zyngier <m...@kernel.org> > Sent: Friday, January 10, 2020 6:35 PM > To: Jianyong Wu <jianyong...@arm.com> > Cc: net...@vger.kernel.org; yangbo...@nxp.com; john.stu...@linaro.org; > t...@linutronix.de; pbonz...@redhat.com; sean.j.christopher...@intel.com; > richardcoch...@gmail.com; Mark Rutland <mark.rutl...@arm.com>; > w...@kernel.org; Suzuki Poulose <suzuki.poul...@arm.com>; Steven Price > <steven.pr...@arm.com>; linux-ker...@vger.kernel.org; linux-arm- > ker...@lists.infradead.org; kvmarm@lists.cs.columbia.edu; > k...@vger.kernel.org; Steve Capper <steve.cap...@arm.com>; Kaly Xin > <kaly....@arm.com>; Justin He <justin...@arm.com>; nd <n...@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. > [...] > > > 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). > Ok, thanks Thanks Jianyong > M. > -- > Jazz is not dead. It just smells funny... _______________________________________________ kvmarm mailing list kvmarm@lists.cs.columbia.edu https://lists.cs.columbia.edu/mailman/listinfo/kvmarm
