Messages in this thread | | | Date | Wed, 24 Sep 2014 09:32:54 -0500 | From | Nathan Lynch <> | Subject | Re: [PATCH v2 0/3] arm_arch_timer: VDSO preparation, code consolidation |
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On 09/24/2014 09:12 AM, Christopher Covington wrote: > Hi Nathan, > > On 09/22/2014 08:28 PM, Nathan Lynch wrote: >> On 09/22/2014 05:30 PM, Russell King - ARM Linux wrote: >>> On Mon, Sep 22, 2014 at 04:39:19PM +0100, Will Deacon wrote: >>>> On Thu, Sep 18, 2014 at 03:59:32PM +0100, Nathan Lynch wrote: >>>>> This series contains the necessary changes to allow architected timer >>>>> access from user-space on 32-bit ARM. This allows the VDSO to support >>>>> high resolution timestamps for clock_gettime and gettimeofday. This >>>>> also merges substantially similar code from arm and arm64 into the >>>>> core arm_arch_timer driver. >>>>> >>>>> The functional changes are: >>>>> - When available, CNTVCT is made readable by user space on arm, as it >>>>> is on arm64. >>>>> - The clocksource name becomes "arch_mem_counter" if CP15 access to >>>>> the counter is not available. >>>>> >>>>> These changes have been carried as part of the ARM VDSO patch set over >>>>> the last several months, but I am splitting them out here as I assume >>>>> they should go through the clocksource maintainers. >>>> >>>> For the series: >>>> >>>> Acked-by: Will Deacon <will.deacon@arm.com> >>>> >>>> I'm not sure which tree the arch-timer stuff usually goes through, but >>>> the arm/arm64 bits look fine so I'm happy for them to merged together. >>> >>> I raised a while back with Will whether there's much point to having >>> this on ARM. While it's useful for virtualisation, the majority of >>> 32-bit ARM doesn't run virtualised. So there's little point in having >>> the VDSO on the majority of platforms - it will just add additional >>> unnecessary cycles slowing down the system calls that the VDSO is >>> designed to try to speed up. >> >> Hmm, this patch set is merely exposing the hardware counter when it is >> present for the VDSO's use; I take it you have no objection to that? >> >> While the 32-bit ARM VDSO I've posted (in a different thread) exploits a >> facility that is required by the virtualization option in the >> architecture, its utility is not limited to guest operating systems. > > Just to clarify, were the performance improvements you measured from a > virtualized guest or native?
Yeah I should have been explicit about this. My tests and measurements (and all test results I've received from others, I believe) have been on native/host kernels, not guests.
>>> So, my view is that this VDSO will only be of very limited use for >>> 32-bit ARM, and should not be exposed to userspace unless there is >>> a reason for it to be exposed (iow, the hardware necessary to support >>> it is present.) >> >> My thinking is that it should prove useful in a growing subset of v7 >> CPUs. It is useful today on Cortex-A15 and -A7, and I believe -A12 and >> -A17 implement the generic timer facility as well. > > I count 18 dts* files that have "arm,armv7-timer", including platforms with > Krait, Exynos, and Tegra processors.
Yup.
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