linux-kernel - Re: [PATCH 3/6] x86-64: Put vsyscall_gtod

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Date:	Mon, 28 Mar 2011 14:09:43 -0400
From:	Andrew Lutomirski <luto@....edu>
To:	Thomas Gleixner <tglx@...utronix.de>
Cc:	x86@...nel.org, linux-kernel@...r.kernel.org,
	John Stultz <johnstul@...ibm.com>
Subject: Re: [PATCH 3/6] x86-64: Put vsyscall_gtod_data at a fixed virtual address

On Mon, Mar 28, 2011 at 1:49 PM, Thomas Gleixner <tglx@...utronix.de> wrote:
> On Mon, 28 Mar 2011, Andy Lutomirski wrote:
>>
>> +      /* This address must be kept in sync with arch/x86/vdso/vextern.h */
>
> This is the worst idea ever. Breakage is lurking by making something:
> Must be kept in sync ...
>
>> +     .vsyscall_gtod_data ADDR(.vsyscall_0) + 3072 + 256:
>
> And this magic constant is what ? Something which will blow up sooner
> than later.

Fair enough.  The constant is pretty much arbitrary, except that it
has to be the same in both places.

Would you object less if I added a new header that just contained
offsets within the vsyscall page to clock, vgetcpu_mode, and
gtod_data?  That way I could remove the entire VMAGIC mechanism,
everything gets faster, and there's nothing to be kept in sync.

I just checked and the linker will error out of any of these data
structures get too large to fit in their assigned places.  It looks
like this:

ld: section .vsyscall_1 loaded at [00000000018ba400,00000000018ba43d]
overlaps section .vsyscall_gtod_data loaded at
[00000000018ba140,00000000018ba5d7]
ld: .tmp_vmlinux1: section .vsyscall_1 vma 0xffffffffff600400 overlaps
previous sections

(That's on an unmodified kernel, but the same thing would happen with
hardcoded offsets.)

--Andy
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