RE: [Xen-devel] 16 GB limit on Xen x86-32 PAE?

["Ian Pratt" <m+Ian.Pratt@xxxxxxxxxxxx>]

> I was trying to find out what the maximum limit for memory is 
> on x86-32 PAE. 
> I discovered that the limit was 16 GB and it was set in 
> xen/asm-x86/config.h
> 
> #define MACHPHYS_MBYTES        16 /* 1 MB needed per 1 GB memory */
> #define FRAMETABLE_MBYTES (MACHPHYS_MBYTES * 6)
> 
> Is there any reason for this 16 GB limit? Doesn't PAE permit 
> us to access up to 64 GB?

Xen currently uses 32 bytes of virtual address per machine page. Above
16GB, this is consuming significant amounts of virtual address space
that is normally available to the guest OS kernel, possibly causing
virtual memory balancing issues.  

> If we wanted to access 64 GB what else would need to be fixed?

It'll probably work, but you're on your own. Enabling CONFIG_HIGHPTE in
Linux will certainly help the balancing issue.

Longer term, if we want to go above 16GB we'll probably squeeze the
pfn_info structure to save some space, but this won't happen for 3.0.

Ian

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