Messages in this thread |  | | Date | Mon, 07 Apr 2008 22:46:30 +0200 | From | Thomas Hellström <> | Subject | Re: [PATCH] x86: create array based interface to change page attribute |
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Jesse Barnes wrote: > On Monday, April 07, 2008 12:51 pm Thomas Hellström wrote: > >>> Hopefully the WC stuff will be upstream right after 2.6.25 comes out. >>> Any reason why we shouldn't keep the pages mapped in the kernel as WC >>> assuming the interface is there? >>> >> If the pages are unmapped, we can get reasonable speed doing >> unbind-read-bind operations, kernel accesses to the memory will need to >> use an iomap_atomic_prot_pfn() type of operation. >> No IPI global tlb flushes needed for kernel mapping changes during >> unbind-read-bind and no cache flushes needed either if we write-protect >> the user-space mappings properly, or very limited cache flushes if we >> keep dirty-written-while-cached flags for each page. >> >> If the pages are wc-d we'll need two extra IPI global tlb flushes and a >> buffer-size cache flush every time we do unbind-read-bind, but OTOH we >> don't need the iomap_atomic_prot_pfn() to access single pages from the >> kernel. >> > > Why would we need to flush at all at unbind-read-bind time? We should be able > to leave pages in the WC state even when we unbind them, then when we need to > bind them back into the GTT they'll be ready, but maybe I'm misunderstanding > you here... > > We want to make the user-space mapping cache-coherent after unbind during read, to have any serious read-speed, and the linear kernel map has to follow, unless it's non-present. Even if it's non present, we need to flush whatever was written through the user-space mapping from the cache when rebinding. Having the user-space mapping read-only when possible will help avoid this. >>> Also, to make the API readable, we'd probably want to split the function >>> into kernel_map_pages(..., enum memory_type type) and >>> kernel_unmap_pages(...) (though like I said I think we really should be >>> mapping them WC not umapping them altogether, since we do want to hit the >>> ring buffer from the kernel with the WC type for example). >>> >> I think ring-buffers are using ioremap() or vmap() already today. We can >> use these to get WC-type access also in the future. The only time we use >> the linear kernel mapping today is for single page access while patching >> up command buffers. >> > > Yeah, they're ioremapped now, but that's a problem since with the PAT patches > they'll be mapped hard UC (right now it just happens to work). > Ouch, so we'll be needing an ioremap_wc(), I guess. We probably shouldn't use the linear kernel map for this anyway, since that would require a chipset flush for each ring commit. We can actually use vmap() with a wc page protection for that, but an ioremap_wc() would certainly save us a lot of trouble. > Thanks, > Jesse > /Thomas
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