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

[Dr. David Alan Gilbert]

* Li, Tianyou (address@hidden) wrote:
> 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 
> user,id=user.0,hostfwd=tcp::5555-:22 
> ${SRC_DIR}/../images/snapshot/arch-snapshot1.img
> 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 
> conv=notrunc
> 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.

I wonder if QEMU or the guest (BIOS? Kernel?)  is zeroing the memory ?  For 
normal memory
I'd expect it to zero it.

Dave

> 
> 
> Regards,
> Tianyou
> 
> -----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. Thanks.
> 
> Regards,
> Tianyou
> 
> -----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:
> http://blog.vmsplice.net/2016/01/qemu-internals-how-guest-physical-ram.html
> 
> Regards,
> Artyom
> 
> 
> --
> Regards,
> Artyom Tarasenko
> 
> SPARC and PPC PReP under qemu blog: http://tyom.blogspot.com/search/label/qemu
--
Dr. David Alan Gilbert / address@hidden / Manchester, UK