linux-kernel - Re: [v3, 3/3] MIPS: VDSO: Add implementations of gettimeofday() and clock

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Date:	Tue, 27 Oct 2015 14:44:58 -0700
From:	David Daney <ddaney.cavm@...il.com>
To:	Leonid Yegoshin <Leonid.Yegoshin@...tec.com>
CC:	Ralf Baechle <ralf@...ux-mips.org>,
	Markos Chandras <markos.chandras@...tec.com>,
	linux-mips@...ux-mips.org, Alex Smith <alex.smith@...tec.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [v3, 3/3] MIPS: VDSO: Add implementations of gettimeofday() and
 clock_gettime()

On 10/27/2015 02:15 PM, Leonid Yegoshin wrote:
> On 10/27/2015 02:02 PM, David Daney wrote:
>> On 10/27/2015 01:46 PM, Leonid Yegoshin wrote:
>> [...]
>>>
>>> And finally. clock scaling - what we would do if there are two CPUs with
>>> different clock ratios in system? It seems like common kernel timing
>>> subsystem can handle that.
>>>
>>
>> The code that executes in userspace must have access to a consistent
>> clock source.  If you are running on a SMP system that doesn't have
>> synchronized CP0.Count registers, then your gettimeofday() cannot use
>> CP0.Count (RDHWR $2).
>
> Right, I agree.
>
>>
>> As far as I know, CP0.Count is the only available counter visible to
>> userspace, so you would have to disable the accelerated versions of
>> gettimeofday() where you cannot assert that the counters are always
>> synchronized.
>
> Any system with GIC may have access to the same GIC global counter in a
> special separate page available for mapping by user in RO mode and it
> seems Alex did that.
>
> Besides that this GIC global counter is used as a major system
> clocksource in systems with GIC.

Yes, I had forgotten about the GIC thing.

In any event, there is a set of systems where we could run into problems 
with unsynchronized clocks.  There needs to be an easy way to 
enable/disable the gettimeofday() acceleration at run time based on the 
properties of the counter (GIC, CP0.Count, or whatever) chosen at boot time.

For example, On OCTEON single-chip systems we synchronize the the 
counters and they don't drift.  So, we can use CPO.Count.  However, on 
two-chip NUMA configurations there may be clock drift between the two 
chips, so CPO.Count cannot be used as a clocksource.  We have a single 
kernel image that runs on both types of systems, so we have to be able 
to enable/disable the gettimeofday() acceleration.

David Daney


>
> - Leonid
>
>
>
>

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