linux-kernel - Re: [PATCH] Add VDSO time function support for x86 32-bit kernel

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Message-ID: <CALCETrVAhncj7R7pCTBqn9erC_fKoWzaO-PFBdf=d6-wQJCzTg@mail.gmail.com>
Date:	Tue, 11 Dec 2012 13:18:42 -0800
From:	Andy Lutomirski <luto@...capital.net>
To:	Stefani Seibold <stefani@...bold.net>
Cc:	John Stultz <john.stultz@...aro.org>, linux-kernel@...r.kernel.org,
	ak@...ux.intel.com, aarcange@...hat.com
Subject: Re: [PATCH] Add VDSO time function support for x86 32-bit kernel

On Tue, Dec 11, 2012 at 12:54 PM, Stefani Seibold <stefani@...bold.net> wrote:
> Am Dienstag, den 11.12.2012, 11:27 -0800 schrieb John Stultz:
>> On 12/11/2012 08:11 AM, stefani@...bold.net wrote:
>> > From: Stefani Seibold <stefani@...bold.net>
>> >
>> > This small patch add the functions vdso_gettimeofday(), vdso_clock_gettime()
>> > and vdso_time() support to the VDSO for x86 32-bit kernels.
>> >
>> > The reason to do this was to get a fast reliable time stamp. Many developers
>> > uses TSC to get a fast time time stamp, without knowing the pitfalls. VDSO
>> > time functions a fast and reliable way, because the kernel knows the best time
>> > source and the P- and C-state of the CPU.
>> Very cool. There have been similar implementations of this patch over
>> the years, but they were all bit more hackish then this.
>>
> Thanks, i think it is not more hackish than the x86 64 code.
>
>>
>> > For x86 the vclock_gettime.c currently supports only the HPET and TSC timer,
>> > the ACPI timer should be easily to add with an other patch.
>> Although the ACPI PM timer requires port-io which would need tweaking to
>> allow normal users to access it. And I'm not sure if the performance
>> would be much improved, as the port-io probably dominates the
>> performance cost.
>>
>
> It was only an idea. I think it would be easy to give the ioperm read
> for the ACPI Timer without breaking anything.

I think you'll have to be careful not to make context switches slower
if you do this.  IIRC ioperm bitmaps live in the task structure (the
i386 thing, not task_struct) and require extra slow instructions.

I still think we should do TSC only.

>> Could you expand a bit as to why a compat layer isn't possible? It seems
>> we could easily convert the vsyscall_gtod_data to a more explicit
>> arch-neutral size. Or is it the actual data page mapping?
>>
>
> This could be done in a subsequent patch. First i want not change to
> much, to make the review easier. Converting the vsyscall_gtod_data to
> arch neutral is not so easy, because the size of time_t, timezone and
> timespec differs. So currently 32 bit programs running under a 64 bit
> kernel will not get a VDSO with time functions. But real 32 bit kernel
> will provide it.
>
>

How would you address the vvar page in 32-bit userspace on a 64-bit
kernel?  Would you map it low for 32-bit tasks or play descriptor
games?

--Andy
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