Messages in this thread | | | Date | Mon, 24 Aug 1998 09:28:39 -0700 (PDT) | From | Linus Torvalds <> | Subject | Re: 64 bit PCI and Linux |
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On Sun, 23 Aug 1998, David S. Miller wrote: > > Keep in mind that Linux will probably never support a >2GB physical > address space, let alone a 36-bit one... The memory management issues > are just too hairy and hard to make work when the physical address > space is larger than the virtual one.
Note that Linux probably _will_ support access to the high physical memory as a buffer cache or similar. It's the only reasonable use for it, and it's the same thing NT does, for example. For once MS did a reasonable thing - they told intel to forget about their original extremely hairy scheme, and the new PAE36 things in the Xeon etc CPU's make it easy to access the high memory without doing the horrible cludgy 3-level page table thing.
(Not that three level page tables are bad - they aren't - but as done by intel they just sucked ;)
So there might actually be interest in using 64-bit PCI on intel, but probably only in a very limited sense: using 64-bit DMA in order to fill and write from the upper memory region. That should be fairly transparent from anything else - it doesn't involve remapping shared areas or anything like that.
[ Side issue: I don't actually know how shared PCI devices show up in a machine with 4GB of memory, so that may be a problem. It's not a problem on non-intel machines, because on the alpha and others the PCI space is completely separated from normal memory, so it should be ok to have memory and PCI mapped at the same address even though it probably means that you lose device-to-device direct transfers ]
Linus
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