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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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