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

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Message-ID: <50CA7486.2070804@zytor.com>
Date:	Thu, 13 Dec 2012 16:36:22 -0800
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Andy Lutomirski <luto@...capital.net>
CC:	Stefani Seibold <stefani@...bold.net>,
	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
Subject: Re: [PATCH] Add VDSO time function support for x86 32-bit kernel

On 12/13/2012 04:20 PM, Andy Lutomirski wrote:
>>
>> Whatever data you need you can just map it into the vdso range.  There
>> really shouldn't be anything special about that at all.
>>
>> The fixmap stuff is an x86-64 legacy that you don't have to worry about,
>> obviously.
>
> The fixmap page is new.  It's not ABI -- the layout can freely change,
> but the vdso needs to be able to see.  It would make no sense to cow
> that page, and it would be more complicated to make it be part of the
> (64-bit) vdso, so I left it as a fixmap when I did the vvar cleanups.
>

Well, the vsyscall fixmap is an ABI.  But just because a page is mapped 
into userspace doesn't mean cow.  Think of it as a device mmap, or an 
mmap of a shared file.

> For compat mode, though, I don't think it can be in the fixmap unless
> there are segmentation games that I think are impossible, or unless
> the vdso wants to do a far call (which I would guess is not much
> faster than a system call, although I haven't benchmarked it).  This
> is because there are no addresses at all that can be seen from 32-bit
> code segments and that are in the kernel address range.

Yes, you'd have to nip into 64-bit mode which is not really practical.

> What you could do is probably arrange (using some linker script magic)
> for a symbol to exist that points at the page *before* the vdso
> starts.  Then just map the vvar page there when starting a compat
> task.  You could then address it using a normal symbol reference by
> tweaking the vvar macro.  (I think this'll access it via the GOT.)
> Alternatively, you could just pick an absolute address -- the page is
> NX, so you don't really need to worry about randomization.

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.

	-hpa

-- 
H. Peter Anvin, Intel Open Source Technology Center
I work for Intel.  I don't speak on their behalf.

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