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Re: [Qemu-devel] Persistent Main Memory in QEmu

From: Li, Tianyou
Subject: Re: [Qemu-devel] Persistent Main Memory in QEmu
Date: Thu, 21 Apr 2016 10:47:50 +0000

Hi Artyom, Dave & others,

An update: I have tried to use -mem-path option to make a file-backed memory. I 
have tried to write a particular physical address in guest Linux OS with 
specific value to verify the persistency characteristics. The result is not as 
we expected. Below is more details.

1. guest os launch: qemu-system-x86_64 -monitor stdio -machine pc -enable-kvm 
-smp 1  -m 8G  -mem-path ${RAM_FILE}  -device e1000,netdev=user.0 -netdev 
2. install fmem kernel module in guest linux, the code is here (quick & dirty 
workaround for mem_write, please ignore): https://github.com/TianyouLi/fmem
3. run printf "worldhello" | dd of=/dev/fmem bs=1 count=10 seek=6294967296 
4. run dd if=/dev/fmem bs=1 count=10 seek=6294967296 of=out
5. cat out   -------> here the result is "worldhello"
6. poweroff ---> guest shutdown
7. re-launch guest use #1
8. run #4, get memory content of same address
9. the result is not as expected, all zero there

I am looking into the code in numa.c of function 
allocate_system_memory_nonnuma, which do init memory region and call 
qemu_ram_mmap in mmap-alloc.c. Seems the mmap with fd has been setup correctly 
so that memory data read/write should be flushed to file and survive next time 
of boot. I am not quite clear why step #9 failed to get the value we previous 
set in step #4. There could be the test methodology problem or can be my 
in-correct understanding of qemu feature. Could you please elaborate more 
details or give me some hints? Thanks.


-----Original Message-----
From: Li, Tianyou 
Sent: Tuesday, April 19, 2016 10:53 AM
To: Artyom Tarasenko <address@hidden>
Cc: address@hidden
Subject: RE: [Qemu-devel] Persistent Main Memory in QEmu

Hi Artyom,

Thanks for your pointer! I have tried the -mem-path option, and right now 
looking into the code to see if the content of the file will be used during the 
guest Linux running next time. Will let you know the result. 

Stefan's post is definitely helpful, thanks for letting me know. Could you 
please let me know if you have more QEmu memory management documentation about 
internals? I have googled around, but more likely to hear from your advice. 


-----Original Message-----
From: Artyom Tarasenko [mailto:address@hidden
Sent: Tuesday, April 19, 2016 4:20 AM
To: Li, Tianyou <address@hidden>
Cc: address@hidden
Subject: Re: [Qemu-devel] Persistent Main Memory in QEmu

Hi Tianyou,

On Mon, Apr 18, 2016 at 5:50 AM, Li, Tianyou <address@hidden> wrote:
> Currently we are trying to implement below functionalities in QEmu: 
> main memory in guest can be logically viewed as persistent and its 
> content can be survived through reboot or shutdown/powerup.
> I have looked into the QEmu memory management code include memory.c, 
> exec.c and other related source, unfortunately I do not have the 
> chance to get clue of how to make QEmu main memory persistent. I found 
> that pmemsave could dump physical memory of guest, but I could not 
> find how to restore the dump file before VM startup to execution.
> Could anyone provide some hints to me? Thanks in advance!

Is the option "-mem-path=/path/to/mem-file" what you are looking for?

Stefan wrote a nice post about QEMU RAM internals:


Artyom Tarasenko

SPARC and PPC PReP under qemu blog: http://tyom.blogspot.com/search/label/qemu

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