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SubjectRe: [git head] X86_PAT & mprotect
On Sat, May 10, 2008 at 6:05 AM, Venki Pallipadi
<venkatesh.pallipadi@intel.com> wrote:
> On Fri, May 09, 2008 at 12:08:18PM +0200, Ingo Molnar wrote:
> >
> > * Venki Pallipadi <venkatesh.pallipadi@intel.com> 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@elte.hu>
> > Tested-by: Ingo Molnar <mingo@elte.hu>
> >
> > 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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