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    SubjectRe: CPA patchset

    * Andi Kleen <> wrote:

    > > What is very real though are the hard limitations of MTRRs. So i'd
    > > rather first like to see a clean PAT approach (which all other
    > > modern OSs have already migrated to in the past 10 years)
    > That's mostly orthogonal. Don't know why you bring it up now?

    because the PAT (Page Attribute Table support) patchset and the CPA
    (change_page_attr()) patchset are are not orthogonal at all - as their
    name already signals: because they change the implementation/effects of
    the same interface(s). [just at different levels].

    Both patchsets change how the kernel pagetable caching is handled. PAT
    changes the kernel pte details and unshackles us from MTRR reliance and
    thus solves real problems on real boxes:

    55 files changed, 1288 insertions(+), 237 deletions(-)

    CPA moves change_page_attr() from invwb flushing to cflush flushing, for
    a speedup/latency-win, plus a whole bunch of intermingled fixes and
    improvements to page attribute modification:

    26 files changed, 882 insertions(+), 423 deletions(-)

    so in terms of risk management, the "perfect patch order" is:

    - minimal_set of correctness fixes to the highlevel cpa code.

    - ( then any provably NOP cleanups to pave the way. )

    - then change the lowlevel pte code (PAT) to reduce/eliminate the need
    to have runtime MTRR use

    - then structural improvements/cleanups of the highlevel cpa code

    - then the cflush (optional) performance feature ontop of it.

    - then gigabyte-largepages/TLBs support [new CPU feature that further
    complicates page-attribute management]

    All in an easy-to-revert fashion. We _will_ regress here, and this stuff
    is very hard to debug.


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