Re: [git head] X86_PAT & mprotect

On Sat, May 10, 2008 at 6:05 AM, Venki Pallipadi
<venkatesh.pallipadi@xxxxxxxxx> wrote:
On Fri, May 09, 2008 at 12:08:18PM +0200, Ingo Molnar wrote:
>
> * Venki Pallipadi <venkatesh.pallipadi@xxxxxxxxx> wrote:
>
> > > I've tried doing it slightly differently below, don't know whether
> > > you'll consider it an improvement or not.
> >
> > Hugh: Thanks for looking into this. Yes. I like your modified patch.
> > Simpler and smaller.
>
> i have stuck your original patch into testing and nothing blew up so
> far. Due to the mm/ bits this is not for the scope of x86.git, but
> obviously it all looks good and is .26-worthy to me:
>
> Acked-by: Ingo Molnar <mingo@xxxxxxx>
> Tested-by: Ingo Molnar <mingo@xxxxxxx>
>
> Venki, could you please send a full patch against -git that has
> everything from Hugh included, with an updated changelog, for
> Linus/Andrew to ack/apply?
>

Ingo,

Split up the patch into two parts as the pci part was unrelated to mprotect
problem in a sense.

Here is the first patch.

Thanks,
Venki


Some versions of X used the mprotect workaround to change caching type from

UC to WB, so that it can then use mtrr to program WC for that region [1].
Change the mmap of pci space through /sys or /proc interfaces from UC to
UC_MINUS. With this change, X will not need to use mprotect
workaround to get WC type.
Also the bug with mprotect which lets caller to change PAT bits is fixed in
the follow on patch. So, this X workaround will stop working as well.


Wow this kinda puts X in a nasty position, we have 2.6.25 and previous kernels
where we use the original /sys interfaces and nasty hack to
workaround, but on 2.6.26 we magically need to
switch to the /sys _uc interfaces or the users X will slow down.

Granted I think only F9 is shipping libpciaccess so far, but now we
need to fix it up and make sure a new one exists before
2.6.26 hits users. Build it yourself users are going to be noticing
the slowdown I suspect.

Dave.
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