Re: [Qemu-devel] [PATCH v9 00/56] Postcopy implementation

[Dr. David Alan Gilbert]

* Bharata B Rao (address@hidden) wrote:
> On Fri, Nov 6, 2015 at 2:39 PM, Dr. David Alan Gilbert
> <address@hidden> wrote:
> > * Bharata B Rao (address@hidden) wrote:
> >> On Thu, Nov 05, 2015 at 06:10:27PM +0000, Dr. David Alan Gilbert (git) 
> >> wrote:
> >> > From: "Dr. David Alan Gilbert" <address@hidden>
> >> >
> >> >   This is the 9th cut of my version of postcopy.
> >> >
> >> > The userfaultfd linux kernel code is now in the upstream kernel
> >> > tree, and so 4.3 can be used without modification.
> >> >
> >> > This qemu series can be found at:
> >> > https://github.com/orbitfp7/qemu.git
> >> > on the wp3-postcopy-v9 tag
> >> >
> >> > Testing status:
> >> >   * Tested heavily on x86
> >> >   * Smoke tested on aarch64 (so it does work on different page sizes)
> >>
> >> Tested minimally on ppc64 with back and forth postcopy migration of
> >> unloaded pseries guest within the localhost - works as expected.
> >>
> >> However I am seeing a failure in one case. I am not sure if this is
> >> a user error or a real issue in postcopy migration. If I switch to postcopy
> >> migration immediately after starting the migration, I see the migration
> >> failing with error:
> >>
> >> qemu-system-ppc64: qemu_savevm_send_packaged: Unreasonably large packaged 
> >> state: 25905005
> >
> > I put an arbitrary limit of 16MB (see MAX_VM_CMD_PACKAGED_SIZE in 
> > include/sysemu/sysemu.h)
> > on the size of the data accepted into the packaged blob.  How big is the 
> > htab data likely to be?
> 
> HTAB size is a variable and depends on maxmem size. It will be 1/128
> th of maxmem. So for a 32G guest, HTAB will be 256M in size.

OK, that does get a bit big.
Two possible fixes;
 1 - postcopy htab (I don't know htab to know how hard that is)
 2 - do one pass of iterable/non-postcopiable devices before we start the 
package;
     I'm just writing a patch to try that; I'll send it to you to let
     you try once I get it to not-break normal migration.

Dave

> 
> Regards,
> Bharata.
--
Dr. David Alan Gilbert / address@hidden / Manchester, UK