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Re: [Qemu-devel] [PATCH v8 4/8] ACPI: Add Virtual Machine Generation ID

From: Dr. David Alan Gilbert
Subject: Re: [Qemu-devel] [PATCH v8 4/8] ACPI: Add Virtual Machine Generation ID support
Date: Mon, 20 Feb 2017 12:32:41 +0000
User-agent: Mutt/1.7.1 (2016-10-04)

* Laszlo Ersek (address@hidden) wrote:
> On 02/20/17 12:00, Dr. David Alan Gilbert wrote:
> > * Laszlo Ersek (address@hidden) wrote:
> >> On 02/20/17 11:23, Dr. David Alan Gilbert wrote:
> >>> * Laszlo Ersek (address@hidden) wrote:
> >>>> CC Dave
> >>>
> >>> This isn't an area I really understand; but if I'm
> >>> reading this right then 
> >>>    vmgenid is stored in fw_cfg?
> >>>    fw_cfg isn't migrated
> >>>
> >>> So why should any changes to it get migrated, except if it's already
> >>> been read by the guest (and if the guest reads it again aftwards what's
> >>> it expected to read?)
> >>
> >> This is what we have here:
> >> - QEMU formats read-only fw_cfg blob with GUID
> >> - guest downloads blob, places it in guest RAM
> >> - guest tells QEMU the guest-side address of the blob
> >> - during migration, guest RAM is transferred
> >> - after migration, in the device's post_load callback, QEMU overwrites
> >>   the GUID in guest RAM with a different value, and injects an SCI
> >>
> >> I CC'd you for the following reason: Igor reported that he didn't see
> >> either the fresh GUID or the SCI in the guest, on the target host, after
> >> migration. I figured that perhaps there was an ordering issue between
> >> RAM loading and post_load execution on the target host, and so I
> >> proposed to delay the RAM overwrite + SCI injection a bit more;
> >> following the pattern seen in your commit 90c647db8d59.
> >>
> >> However, since then, both Ben and myself tested the code with migration
> >> (using "virsh save" (Ben) and "virsh managedsave" (myself)), with
> >> Windows and Linux guests, and it works for us; there seems to be no
> >> ordering issue with the current code (= overwrite RAM + inject SCI in
> >> the post_load callback()).
> >>
> >> For now we don't understand why it doesn't work for Igor (Igor used
> >> exec/gzip migration to/from a local file using direct QEMU monitor
> >> commands / options, no libvirt). And, copying the pattern seen in your
> >> commit 90c647db8d59 didn't help in his case (while it wasn't even
> >> necessary for success in Ben's and my testing).
> > 
> > One thing I noticed in Igor's test was that he did a 'stop' on the source
> > before the migate, and so it's probably still paused on the destination
> > after the migration is loaded, so anything the guest needs to do might
> > not have happened until it's started.
> Interesting! I hope Igor can double-check this!
> In the virsh docs, before doing my tests, I read that "managedsave"
> optionally took --running or --paused:
>     Normally, starting a managed save will decide between running or
>     paused based on the state the domain was in when the save was done;
>     passing either the --running or --paused flag will allow overriding
>     which state the start should use.
> I didn't pass any such flag ultimately, and I didn't stop the guests
> before the managedsave. Indeed they continued execution right after
> being loaded with "virsh start".
> (Side point: managedsave is awesome. :) )

If I've followed the bread crumbs correctly, I think managedsave
is just using a migrate to fd anyway, so the same code.

> > 
> > You say;
> >    'guest tells QEMU the guest-side address of the blob'
> > how is that stored/migrated/etc ?
> It is a uint8_t[8] array (little endian representation), linked into
> another (writeable) fw_cfg entry, and it's migrated explicitly (it has a
> descriptor in the device's vmstate descriptor). The post_load callback
> relies on this array being restored before the migration infrastructure
> calls post_load.

RAM normally comes back before other devices, so you should be OK;
although we frequently have problems with devices reading from RAM
during device init before migration has started, or writing to it
after migration has finished on the source.


> Thanks
> Laszlo
Dr. David Alan Gilbert / address@hidden / Manchester, UK

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