Date: Thu, 29 Jul 2010 15:02:56 -0500 (CDT) From: Sergey Babkin <babkin@verizon.net> To: avg@icyb.net.ua Cc: freebsd-hackers@freebsd.org, pebu3op@googlemail.com Subject: Re: Re: coherence-problem on the mapped memory buffer Message-ID: <382607918.1356296.1280433776963.JavaMail.root@vms170009.mailsrvcs.net>
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Jul 29, 2010 12:58:07 PM, avg@icyb.net.ua wrote: >on 29/07/2010 19:13 Andriy Gapon said the following: >> on 29/07/2010 17:13 Alexander Fiveg said the following: >In fact I have a suspicion that the problem might have to do with multiple >mappings of the shared pages, but far from sure... >Take a look at Intel® 64 and IA-32 Architectures Software Developer’s Manual >Volume 3A - System Programming Guide, Part 1; Chapter 11.12.4 Programming the PAT; >starting at the following words: >«The PAT allows any memory type to be specified in the page tables, and therefore >it is possible to have a single physical page mapped to two or more different >linear addresses, each with different memory types. Intel does not support this >practice...» My guess would be that the memory type is not marked as DMA-capable. AFAIK the Intel CPUs do the hardware snooping on the physical addresses, so they have no coherency issues benween themselves. However if a DMA writer changes the memory, this I think does not get normally propagated to the front-side bus, and the CPUs would not see it. You may need to either explicitly flush the CPU cache before accessing these pages or mark them as non-cacheable. -SB
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