Messages in this thread | | | Date | Thu, 27 Mar 2008 18:30:27 +0100 | From | Andreas Herrmann <> | Subject | Re: [PATCH] - Increase max physical memory size of x86_64 |
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On Tue, Mar 25, 2008 at 05:02:46PM -0400, Chris Snook wrote: > Jack Steiner wrote: >> On Tue, Mar 25, 2008 at 05:41:54PM +0100, Andreas Herrmann wrote: >>> On Fri, Mar 21, 2008 at 08:31:57AM -0500, Jack Steiner wrote: >>>> Increase the maximum physical address size of x86_64 system >>>> to 44-bits. This is in preparation for future chips that >>>> support larger physical memory sizes. >>> Shouldn't this be increased to 48? >>> AMD family 10h CPUs actually support 48 bits for the >>> physical address. >> You are probably correct but I don't work with AMD processors >> and don't understand their requirements. If someone >> wants to submit a patch to support larger phys memory sizes, >> I certainly have no objections.... > > The only advantage 44 bits has over 48 bits is that it allows us to > uniquely identify 4k physical pages with 32 bits, potentially allowing for > tighter packing of certain structures. Do we have any code that does this, > and if so, is it a worthwhile optimization?
I've checked where those defines are used. If I didn't miss something MAX_PHYSADDR_BITS isn't used at all on x86 and MAX_PHYSMEM_BITS is used (directly or indirectly) in several other macros.
But basically it's just section_to_node_table which would increase to 2 or 4 MB depending on MAX_NUMNODES. Using 44 bits this table is just 128 kB resp. 256 kB in size.
> Personally, I think we should support the full capability of the hardware, > but I don't have a 17 TB Opteron box to test with.
I don't have one either. By adjusting some NB-registers it might be possible to configure physical addresses larger than 40 or 44 bits though. (Even if the machine has not more than 1 or 16 TB.) I'll verify whether this is really possible.
At the moment I think it's best to leave the define as is (44 or 40 bit) as there is currently no practical benefit from increasing it to 48 bit.
Regards,
Andreas
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