Messages in this thread | | | From | "Pallipadi, Venkatesh" <> | Date | Tue, 31 Mar 2009 20:17:43 -0700 | Subject | RE: PAGE_CACHE_WC strikes again |
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>-----Original Message----- >From: Jesse Barnes [mailto:jbarnes@virtuousgeek.org] >Sent: Tuesday, March 31, 2009 6:13 PM >To: Siddha, Suresh B >Cc: Pallipadi, Venkatesh; Eric Anholt; lkml >Subject: Re: PAGE_CACHE_WC strikes again > >On Tue, 31 Mar 2009 17:03:10 -0800 >Suresh Siddha <suresh.b.siddha@intel.com> wrote: > >> On Tue, 2009-03-31 at 17:29 -0700, Pallipadi, Venkatesh wrote: >> > 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). >> >> And the driver should use right API to track the underlying >page frame >> thats getting mapped by this vma, with the corresponding attribute. >> API's like remap_pfn_range(), vm_insert_pfn() will setup the PTE's >> aswell as track the pfn's attributes. >> >> API's like set_memory_uc/wc() will explicitly setup the page >> attributes. >> >> Jesse, As far as I see, the drm GEM fault handler routines don't seem >> to do any of this. Am I missing something? We need to fix this so >> that we can avoid potential aliasing issues. > >Right, the drm driver code went in before we had pgprot_writecombine. >Now that it's available we should definitely use it. I'm not sure >about the set_memory_* routines though; we create io mappings in >i915_dma.c at init time, and I thought we took care of things in >i915_gem.c but we may need updates there. >
There is one problem of too many single page mappings, most likely coming from gem vm_insert_pfn. As in this mail http://marc.info/?l=linux-kernel&m=123845244713183&w=2
The actual 'freeing invalid memtype' is a bug that I still trying to zero in on. Regardless of that, having so many small WC mappings in IO region will result it pretty slow vm_insert_pfn. I think we should switch to io_mapping_create_wc kind of API where we do bulk reserve and have a vm_insert_pfn_quick().
Thanks, Venki
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