linux-kernel - Re: [PATCH 6/6] Add 32 bit VDSO support for 32 and 64 bit kernels

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Date:	Tue, 18 Dec 2012 12:37:10 -0800
From:	Andy Lutomirski <luto@...capital.net>
To:	Stefani Seibold <stefani@...bold.net>
Cc:	"H. Peter Anvin" <hpa@...or.com>, linux-kernel@...r.kernel.org,
	x86@...nel.org, tglx@...utronix.de, mingo@...hat.com,
	ak@...ux.intel.com, aarcange@...hat.com, john.stultz@...aro.org,
	xemul@...allels.com, gorcunov@...nvz.org,
	andriy.shevchenko@...ux.intel.com
Subject: Re: [PATCH 6/6] Add 32 bit VDSO support for 32 and 64 bit kernels

On Tue, Dec 18, 2012 at 12:32 PM, Stefani Seibold <stefani@...bold.net> wrote:
> Am Dienstag, den 18.12.2012, 10:44 -0800 schrieb H. Peter Anvin:
>> On 12/18/2012 08:52 AM, Stefani Seibold wrote:
>> >
>> > Pardon, i never disregarded nor i have agreed that this is going to be a
>> > part of the VDSO. I currently have also no idea how to do this and i see
>> > no need at the moment to do this revamp. The 64 bit VDSO lives since
>> > more than 6 years with this kind of implementation.
>> >
>>
>> It was part of this discussion thread, about how to best manage the
>> address space.  Fixed addresses are a major problem, and introducing new
>> ones are extremely undesirable.
>>
>
> There is no introduce of new fix address. There are still there for
> x86_64. If this will currently not a major problem on this architecture
> than it will not for x86_32 too.

Not necessarily true.  On x86-64 (non-compat) the fixmap address is in
kernel space (high bit set), so it can't conflict with anything in
user space.  On true 32-bit mode, the same applies.  In compat mode,
the fixed address is in *user* space and might conflict with existing
uses.

>
>> Hence I wrote:
>>
>> > IMO it seems this is making it way more complicated than it is. Just
>> > make sure you have a section in the vdso where you can map in a data
>> > page with the symbols in the right offsets. Extra points for doing
>> > magic so that it is at the beginning or end, but I think that might
>> > be harder than necessary.
>>
>> Basically, make the vvar and hpet pages part of the vdso page list.
>> Optionally they can be mapped without the MAYWRITE option -- in fact, we
>> could easily split the vdso into an executable area which gets MAYWRITE
>> to be able to set breakpoints and a data area which doesn't -- but that
>> is a minor tweak IMO.
>>
>
> I see the benefits, but it will not work under all circumstance. The
> VDSO compat mode for x86_32 requires a fix address and there is no room
> behind this. So since this must preserved, i see no real gain for this.

Not true.  It can be mapped with the vdso at a variable address using
GOTOFF addressing.  See my earlier email with
__attribute__((visibility("hidden")).

--Andy
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