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Re: [Xen-devel] [PATCH] xen: reset creation_finished flag on soft reset

Paul Durrant <Paul.Durrant@xxxxxxxxxx> writes:

>> -----Original Message-----
>> From: Vitaly Kuznetsov [mailto:vkuznets@xxxxxxxxxx]
>> Sent: 01 September 2017 10:27
>> To: Andrew Cooper <Andrew.Cooper3@xxxxxxxxxx>; Paul Durrant
>> <Paul.Durrant@xxxxxxxxxx>
>> Cc: xen-devel@xxxxxxxxxxxxx; George Dunlap <George.Dunlap@xxxxxxxxxx>;
>> Ian Jackson <Ian.Jackson@xxxxxxxxxx>; Jan Beulich <jbeulich@xxxxxxxx>;
>> Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>; Stefano Stabellini
>> <sstabellini@xxxxxxxxxx>; Tim (Xen.org) <tim@xxxxxxx>; Wei Liu
>> <wei.liu2@xxxxxxxxxx>
>> Subject: Re: [PATCH] xen: reset creation_finished flag on soft reset
>> Andrew Cooper <andrew.cooper3@xxxxxxxxxx> writes:
>> > On 01/09/2017 10:11, Vitaly Kuznetsov wrote:
>> >> C/s e7dabe5 ("x86/hvm: don't unconditionally create a default ioreq
>> >> server") broke soft reset when QEMU traditional is being used. During
>> >> soft reset QEMU is relaunched and default ioreq server needs to be
>> >> re-created upon first HVM_PARAM_*IOREQ_* request. The flag will be
>> >> set back to 'true' when toolstack unpauses the domain, just like after
>> >> normal creation.
>> >>
>> >> Signed-off-by: Vitaly Kuznetsov <vkuznets@xxxxxxxxxx>
>> >
>> > Sorry, but nack.  d->creation_finished is used for a number of things,
>> > one being TLB safety before the vcpus have started executing.
>> >
>> > We either need to split the variable, or rework e7dabe5 to not use this.
>> >
>> I think that adding another flag is a bad idea, even 'creation_finished'
>> flag looks a bit hackish to me. Adjusting e7dabe5 is probably
>> better. However, while reading its blurb I don't fully understand the
>> change: on migration we create new domain and thus reset
>> creation_finished. During QEMU launch we still need to create ioreq
>> server. Paul, could you please elaborate a bit (e.g. what are we
>> guarding against, when creating ioreq server is redundant) so we can
>> suggest a fix for soft reset?
> My memory is hazy as to the exact problem, but I think it was an issue
> with the COLO project. IIRC they repeatedly 'migrate' a VM but then
> resume the original. Without e7dabe5 the sending VM ends up with a
> default ioreq server after the first migration because the save code
> reads the HVM params that trigger its creation.
> I wonder whether the easiest thing to do would be to modify qemu trad
> to do explicit ioreq server creation? It's really not that much
> code-change... 20-30 lines or so.

I was thinking about this too, I'll try. It will hopefuly allow to get
rid of the 'side effect' which creates default ioreq server on HVM
parameters read.



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