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Re: [Qemu-devel] broken incoming migration

From: Alexey Kardashevskiy
Subject: Re: [Qemu-devel] broken incoming migration
Date: Mon, 03 Jun 2013 20:04:22 +1000
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:17.0) Gecko/20130514 Thunderbird/17.0.6

On 05/31/2013 12:38 AM, Peter Lieven wrote:
> Am 30.05.2013 um 15:41 schrieb "Paolo Bonzini" <address@hidden>:
>> Il 30/05/2013 11:08, Peter Lieven ha scritto:
>>> Am 30.05.2013 10:18, schrieb Alexey Kardashevskiy:
>>>> On 05/30/2013 05:49 PM, Paolo Bonzini wrote:
>>>>> Il 30/05/2013 09:44, Alexey Kardashevskiy ha scritto:
>>>>>> Hi!
>>>>>> I found the migration broken on pseries platform, specifically, this 
>>>>>> patch
>>>>>> broke it:
>>>>>> f1c72795af573b24a7da5eb52375c9aba8a37972
>>>>>> migration: do not sent zero pages in bulk stage
>>>>>> The idea is not to send zero pages to the destination guest which is
>>>>>> expected to have 100% empty RAM.
>>>>>> However on pseries plaftorm the guest always has some stuff in the RAM 
>>>>>> as a
>>>>>> part of initialization (device tree, system firmware and rtas (?)) so it 
>>>>>> is
>>>>>> not completely empty. As the source guest cannot detect this, it skips 
>>>>>> some
>>>>>> pages during migration and we get a broken destination guest. Bug.
>>>>>> While the idea is ok in general, I do not see any easy way to fix it as
>>>>>> neither QEMUMachine::init nor QEMUMachine::reset callbacks has 
>>>>>> information
>>>>>> about whether we are about to receive a migration or not (-incoming
>>>>>> parameter) and we cannot move device-tree and system firmware
>>>>>> initialization anywhere else.
>>>>>> ram_bulk_stage is static and cannot be disabled from the platform
>>>>>> initialization code.
>>>>>> So what would the community suggest?
>>>>> Revert the patch. :)
>>>> I'll wait for 24 hours (forgot to cc: the author) and then post a revert
>>>> patch :)
>>> does this problem only occur on pseries emulation?
>> Probably not.  On a PC, it would occur if you had 4K of zeros in the
>> source BIOS but not in the destination BIOS.  When you reboot, the BIOS
>> image is wrong.
>>> not sending zero pages is not only a performance benefit it also makes
>>> overcomitted memory usable. the madv_dontneed seems to kick in 
>>> asynchronously
>>> and memory is not available immediately.
>> You could also scan the page for nonzero values before writing it.
> i had this in mind, but then choosed the other approach.... turned out to be 
> a bad idea.
> alexey: i will prepare a patch later today, could you then please verify it 
> fixes your problem.

Yes I can, where is the patch? :)

> paolo: would we still need the madvise or is it enough to not write the 
> zeroes?
> Peter
>> Paolo


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