Re: [Qemu-devel] QEMU savevm RAM page offsets

[Laszlo Ersek <lersek@redhat.com>]

On 08/13/13 21:06, Juerg Haefliger wrote:
> On Tue, Aug 13, 2013 at 8:07 PM, Paolo Bonzini <pbonzini@redhat.com> wrote:
>> Il 13/08/2013 19:52, Juerg Haefliger ha scritto:
>>> I didn't mean to imply that the savevm format is broken and needed
>>> fixing. I was just wondering if the data is there and I simply hadn't
>>> found it. Upgrading QEMU is not an option at the moment since these
>>> are tightly controlled productions machines. Is it possible to loadvm
>>> a savevm file from 1.0 with 1.6 to then use guest-memory-dump?
>>
>> Yes, it should, but one important thing since 1.0 has been the merger of
>> qemu-kvm and QEMU.  What distribution are you using?  I know Fedora
>> allows qemu-kvm-1.0 to QEMU-1.6 compatibility, but I don't know about
>> others.
> 
> Ubuntu 12.04
> 
> 
>> Michael Tokarev is the maintainer of the Debian package, so he may be
>> able to answer.
>>
>> Alternatively, you can modify your utility to simply add 512 MB to the
>> addresses above 3.5 GB.
> 
> Is it really as simple as that? Isn't the OS (particularly Windows)
> possibly doing some crazy remapping that needs to be taken into
> account? meminfo on a VM with 4GB running Windows 2008 shows the
> following:
> 
> C:\Users\Administrator\Desktop\MemInfo\amd64>MemInfo.exe -r
> MemInfo v2.10 - Show PFN database information
> Copyright (C) 2007-2009 Alex Ionescu
> www.alex-ionescu.com
> 
> Physical Memory Range: 0000000000001000 to 000000000009B000 (154 pages, 616 KB)
> Physical Memory Range: 0000000000100000 to 00000000DFFFD000 (917245
> pages, 3668980 KB)
> Physical Memory Range: 0000000100000000 to 0000000120000000 (131072
> pages, 524288 KB)
> MmHighestPhysicalPage: 1179648

That should be fine, I think. The 384 KB hole between 640KB and 1MB is
actually contiguously backed by RAMBlock, it is just not (necessarily)
presented as conventional memory to the guest. You can treat the [0,
0x0e0000000) left-closed, right-open interval as contiguous.

Again, check out the diagram in 4/4 that I linked before. Compare it to
pc_init1() in "hw/pc_piix.c", at tag "v1.0" in
<git://git.kernel.org/pub/scm/virt/kvm/qemu-kvm.git>. Look for the
variable "below_4g_mem_size".

Laszlo