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

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Message-ID: <CALCETrWmYpEh9pLiohF+yu9FnrVfSNdHB_TQOKL6+LkdTF+cVw@mail.gmail.com>
Date:	Thu, 13 Dec 2012 11:32:16 -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
Subject: Re: [PATCH] Add VDSO time function support for x86 32-bit kernel

On Wed, Dec 12, 2012 at 11:17 PM, Stefani Seibold <stefani@...bold.net> wrote:
> Am Mittwoch, den 12.12.2012, 22:47 -0800 schrieb H. Peter Anvin:
>> Should be a simple matter of sharing pages.  Look perhaps at the x32 vdso for a hint.
>>
>
>
>> >
>> >Any idea or clean solution how i can map the 64 bit vgtod into the 32
>> >bit address space? Thats the only problem i see.
>> >
>
> No, i see no special handling for x32 vdso to do this. I am not sure if
> x32 vdso can access the 64 bit address space of vsyscall_gtod_data. I
> can't test this due the lack of a x32 abi system.
>
>

x32's vdso cheats -- x32 code can see high addresses just fine.  The
toolchain just makes it difficult.

Your best bet is probably to just map the vvar page twice -- once at
the same address as native 32-bit mode (but only for compat tasks)
would use and once in the usual fixmap location.  You can't use the
fixmap for the compat mapping, though, since it would be a *user*
address.

For HPET support, you'd have to have special support.  I'd say to skip
it for compat mode.

--Andy

-- 
Andy Lutomirski
AMA Capital Management, LLC
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