linux-kernel - Re: [PATCH 3/3] x86, vdso32: handle 32 bit vDSO larger one page

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Date:	Thu, 13 Mar 2014 11:32:36 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	"H. Peter Anvin" <hpa@...or.com>
Cc:	Stefani Seibold <stefani@...bold.net>,
	Greg KH <gregkh@...uxfoundation.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	X86 ML <x86@...nel.org>, Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	Andi Kleen <ak@...ux.intel.com>,
	Andrea Arcangeli <aarcange@...hat.com>,
	John Stultz <john.stultz@...aro.org>,
	Pavel Emelyanov <xemul@...allels.com>,
	Cyrill Gorcunov <gorcunov@...nvz.org>,
	andriy.shevchenko@...ux.intel.com,
	Martin Runge <Martin.Runge@...de-schwarz.com>,
	Andreas Brief <Andreas.Brief@...de-schwarz.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH 3/3] x86, vdso32: handle 32 bit vDSO larger one page

On Thu, Mar 13, 2014 at 11:21 AM, H. Peter Anvin <hpa@...or.com> wrote:
> On 03/13/2014 11:08 AM, H. Peter Anvin wrote:
>> On 03/13/2014 10:28 AM, Andy Lutomirski wrote:
>>>
>>> Does this mean you prefer the relocation approach to the compat vdso
>>> removal approach?  It seems like Linus is okay with either one.
>>>
>>
>> Actually, thinking about it, removing it is probably better:
>>
>> a) gets rid of legacy code, making room for unification;
>> b) either way enabling compat support (either relocation or disabling
>>    the vdso) has a performance penalty for *all* processes.
>>
>> The only way to avoid that is to have a vdso at a fixed addresses across
>> all processes, either in the fixmap or in the user area (presumably at
>> the very top.)
>>
>
> So going back and re-reading all the threads, the consensus was to
> remove the compat vdso, but recycling the CONFIG_COMPAT_VDSO
> configuration option name for the default-disable option.
>

I don't recall anyone suggesting keeping the name.  CONFIG_COMPAT_VDSO
will henceforth be a silly name, since the "compat" mode won't have a
vdso.

The consensus was that the default should be no vdso, right?  My most
recent patchset calls it CONFIG_ENABLE_VDSO32_BY_DEFAULT and defaults
to n.

--Andy

> It is important that anyone who actually cares about performance unsets
> the option.

Should there be a warning at startup if the option is set?

--Andy
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