RE: [git head] X86_PAT & mprotect

[Pallipadi, Venkatesh]

>-----Original Message-----
>From: Dave Airlie [mailto:airlied@xxxxxxxxx] 
>Sent: Friday, May 09, 2008 3:11 PM
>To: Pallipadi, Venkatesh
>Cc: Ingo Molnar; Hugh Dickins; Frans Pop; Barnes, Jesse; 
>linux-kernel@xxxxxxxxxxxxxxx; Packard, Keith; Yinghai Lu; 
>Andrew Morton; Linus Torvalds; H. Peter Anvin; Thomas 
>Gleixner; Nick Piggin; Jesse Barnes
>Subject: 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.
>

No. With 2.6.26 you can still use same /sys resource file, without
the mprotect workaround and set the MTRR as before.

The change from UC to UC_MINUS in this patch ensures that the
X drivers MTRR will take precedence and X does not need any mprotect
hacks.

Thanks,
Venki
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