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RE: [Xen-devel] Yet another "Why can't I see all my RAM" question

To: "Christopher S. Aker" <caker@xxxxxxxxxxxx>, "xen-devel" <xen-devel@xxxxxxxxxxxxxxxxxxx>
Subject: RE: [Xen-devel] Yet another "Why can't I see all my RAM" question
From: "Ian Pratt" <m+Ian.Pratt@xxxxxxxxxxxx>
Date: Tue, 25 Apr 2006 22:43:09 +0100
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Thread-topic: [Xen-devel] Yet another "Why can't I see all my RAM" question
> I know this has been asked a million times, and the typical 
> answer is: 
> that specific area of unallocable memory is used for ACPI 
> data and framebuffers.  However, I've got two identical 
> machines, one running Xen-PAE, one running vanilla 2.6.16:
> Vanilla:
> MemTotal:     16623588 kB
> Xen(PAE)
> total_memory: 14336 (MB)
> I was a little disappointed to discover that my investment in 
> RAM is going to be underutilized in Xen.  I would think if 
> vanilla Linux can utilize all the available RAM that Xen 
> would be able to, also.

PAE Xen currently won't see memory mapped above 16GB. Native Linux
doesn't have this restriction, it sees memory up to 64GB.

It would actually be fairly easy to make the Xen restriction "16GB plus
a bit" so that you'd get the full 16GB of RAM even with a large MMIO
whole in the address space. We might implement that, but using a 64b
hypervisor is definitely the preferred way forward.

> (XEN) WARNING: Only the first 16 GB of the physical memory 
> map can be accessed
> (XEN)          by Xen in 32-bit mode. Truncating the memory map...
> (XEN) Physical RAM map:
> (XEN)  0000000000000000 - 000000000009fc00 (usable)
> (XEN)  000000000009fc00 - 00000000000a0000 (reserved)
> (XEN)  00000000000e8000 - 0000000000100000 (reserved)
> (XEN)  0000000000100000 - 000000007fff0000 (usable)
> (XEN)  000000007fff0000 - 000000007ffff000 (ACPI data)
> (XEN)  000000007ffff000 - 0000000080000000 (ACPI NVS)
> (XEN)  00000000ff780000 - 0000000100000000 (reserved)
> (XEN)  0000000100000000 - 0000000400000000 (usable)
> (XEN) System RAM: 14335MB (14679612kB)
> (XEN) Xen heap: 9MB (10044kB)
> (XEN) Using scheduler: Simple EDF Scheduler (sedf)
> (XEN) PAE enabled, limit: 16 GB
> (XEN) found SMP MP-table at 000ff780
> (XEN) DMI 2.3 present.
> (XEN) Using APIC driver default
> (XEN) ACPI: RSDP (v002 ACPIAM                                
> ) @ 0x000f9760
> (XEN) ACPI: XSDT (v001 A M I  OEMXSDT  0x08000508 MSFT 
> 0x00000097) @ 0x7fff0100
> (XEN) ACPI: FADT (v003 A M I  OEMFACP  0x08000508 MSFT 0x00000097) @
> 0x7fff0281
> (XEN) ACPI: MADT (v001 A M I  OEMAPIC  0x08000508 MSFT 
> 0x00000097) @ 0x7fff0380
> (XEN) ACPI: SPCR (v001 A M I  OEMSPCR  0x08000508 MSFT 
> 0x00000097) @ 0x7fff0410
> (XEN) ACPI: OEMB (v001 A M I  OEMBIOS  0x08000508 MSFT 
> 0x00000097) @ 0x7ffff040
> (XEN) ACPI: SRAT (v001 A M I  OEMSRAT  0x08000508 MSFT 
> 0x00000097) @ 0x7fff3550
> (XEN) ACPI: DSDT (v001  H8DAR H8DAR010 0x00000000 INTL 
> 0x02002026) @ 0x00000000 ...
> (XEN)  Dom0 alloc.:   0000000007800000->0000000008000000 
> (63488 pages to 
> be allocated)
> (XEN)  Loaded kernel: c0100000->c06152e4
> (XEN)  Init. ramdisk: c0616000->c0616000
> (XEN)  Phys-Mach map: c0616000->c0656000
> (XEN)  Start info:    c0656000->c0657000
> (XEN)  Page tables:   c0657000->c0660000
> (XEN)  Boot stack:    c0660000->c0661000
> (XEN)  TOTAL:         c0000000->c0800000
> (XEN)  ENTRY ADDRESS: c0100000
> So what's the scoop here?  Has anyone actually been able to 
> use the full 16GB in PAE mode?  Is there something I can 
> tweak to get that final 1GB back?
> Thanks,
> -Chris
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