Re: [PATCH v2] x86: Remove compat vdso support

[Andy Lutomirski]

On Tue, Mar 11, 2014 at 9:42 AM, H. Peter Anvin <hpa@xxxxxxxxxxxxxxx> wrote:
> On 03/11/2014 09:30 AM, Linus Torvalds wrote:
>>
>> No, the trivial solution is to stop adding crap to it.
>>
>> And no, "just reserve a little more space for it" is neither trivial
>> nor a good idea. The fixed VDSO address is very much at the top of the
>> address space, so you can't allocate more space for it unless you do
>> one of
>>
>>  (a) make it non-contiguous
>>  (b) get rid of the hole that is the very last page
>>  (c) mess with the vsyscall pages and make it contiguous "backwards"
>>
>> all of which sound like *horrible* ideas. Certainly not "trivial solution".
>>
>> No, the trivial solution is to not mess with that legacy page at all.
>>
>> Why is *that* trivial solution not on the table? Why the heck are
>> people hell-bent on changing this stupid legacy page around?
>>
>> I find this whole thread very annoying. We shouldn't care about
>> x86-32, and certainly not from a performance angle - we should
>> consider it a "it's done, don't touch it" issue.
>>
>
> Andy actually did the research, and found that even the legacy VDSO
> doesn't have to live at any one particular address, it just has to live
> at the address it is linked at.  So we can move it just fine, but we
> have to change the link address to match.
>
> That gives us a lot more maneuvering room than saying it has to be at
> one specific address.
>

We could even just relocate the damn thing wherever it ends up.  That
will waste one page of memory per process, though.

--Andy
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