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    SubjectRe: PAGE_CACHE_WC strikes again
    On Tue, 31 Mar 2009 17:29:02 -0700
    "Pallipadi, Venkatesh" <venkatesh.pallipadi@intel.com> wrote:

    >
    >
    > >-----Original Message-----
    > >From: Jesse Barnes [mailto:jbarnes@virtuousgeek.org]
    > >Sent: Tuesday, March 31, 2009 5:15 PM
    > >To: Eric Anholt
    > >Cc: lkml; Pallipadi, Venkatesh
    > >Subject: Re: PAGE_CACHE_WC strikes again
    > >
    > >On Tue, 31 Mar 2009 17:10:47 -0700
    > >Eric Anholt <eric@anholt.net> wrote:
    > >
    > >> I just tracked down what was cutting performance 10x on one of my
    > >> systems on a microbenchmark I'd just written:
    > >>
    > >> --- a/drivers/gpu/drm/drm_gem.c
    > >> +++ b/drivers/gpu/drm/drm_gem.c
    > >> @@ -540,7 +540,7 @@ int drm_gem_mmap(struct file *filp, struct
    > >> vm_area_struct *vma)
    > >> /* FIXME: use pgprot_writecombine when available */
    > >> prot = pgprot_val(vma->vm_page_prot);
    > >> #ifdef CONFIG_X86
    > >> - prot |= _PAGE_CACHE_WC;
    > >> + /*prot |= _PAGE_CACHE_WC;*/
    > >> #endif
    > >> vma->vm_page_prot = __pgprot(prot);
    > >>
    > >> Turns out that setting PAGE_CACHE_WC disables the WC effect of the
    > >> MTRR on my non-PAT (disabled due to CPU errata) 945GM
    > >system, and this
    > >> workaround took GTT-mapped writes from 120MB/s to 1180MB/s.
    > >
    > >What the... There's a pgprot_writecombine now, but it basically does
    > >the same thing. Why is WC so broken? Venki is the fix for this
    > >covered in your last patchset?
    > >
    >
    > The key point here is
    >
    > > setting PAGE_CACHE_WC disables the WC effect of the
    > > MTRR on my non-PAT (disabled due to CPU errata)
    >
    > When PAT is disabled, the default setting in PAT MSR is
    > 00 - WB
    > 01 - WT
    > 10 - UC_MINUS
    > 11 - UC
    >
    > There is no way to set WC with PAT. By hardcoding _PAGE_CACHE_WC
    > (which is 01) the driver is basically selecting write-through!
    >
    > And when MTRR says WC and PAT says WT, effective type is UC.
    >
    > Basically, no one should be hard-coding the memory type. Please use
    > pgprot_writecombine() which does the right thing by using WC
    > (when PAT is enabled) or UC_MINUS (when PAT is disabled).

    Ah ok, that makes sense. I'm glad we have pgprot_writecombine now
    (sorry Eric I should have sent that patch along with my patch
    containing the vm_insert_pfn -EINVAL handling).

    --
    Jesse Barnes, Intel Open Source Technology Center


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