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

From: Laszlo Ersek
Subject: Re: [Qemu-devel] [PATCH v8 4/8] ACPI: Add Virtual Machine Generation ID support
Date: Mon, 20 Feb 2017 16:35:00 +0100
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:45.0) Gecko/20100101 Thunderbird/45.7.1

So Igor has now confirmed he's fine with v8 (thanks!), but I still
wanted to respond here:

On 02/20/17 13:32, Dr. David Alan Gilbert wrote:
> * 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.

Yes, I agree.

My enthusiasm for "managedsave" is due to "virsh start"'s awareness as
to whether it should boot the guest from zero, or in-migrate it from the
"managed" saved state.

Plain "save" is much more risky for the admin to mess up (because it
needs specialized guest startup too).

Of course, I also find QEMU's migration feature awesome in the first
place. :)

>>> 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; we should be fine then. (We only write to RAM in post_load.)


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