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From: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
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Date: Wed, 6 Jan 2016 21:17:59 +0000
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Subject: Re: [Qemu-devel] [PATCH 2/4] macio: add dma_active to
 VMStateDescription
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To: John Snow <jsnow@redhat.com>, qemu-devel@nongnu.org, qemu-ppc@nongnu.org, agraf@suse.de
Cc: Stefan Hajnoczi <stefanha@redhat.com>

On 06/01/16 20:57, John Snow wrote:

> On 01/06/2016 03:37 PM, Mark Cave-Ayland wrote:
>> Make sure that we include the value of dma_active in the migration stream.
>>
>> Signed-off-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
>> ---
>>  hw/ide/macio.c |    3 ++-
>>  1 file changed, 2 insertions(+), 1 deletion(-)
>>
>> diff --git a/hw/ide/macio.c b/hw/ide/macio.c
>> index 560c071..695d4d2 100644
>> --- a/hw/ide/macio.c
>> +++ b/hw/ide/macio.c
>> @@ -518,11 +518,12 @@ static const MemoryRegionOps pmac_ide_ops = {
>>  
>>  static const VMStateDescription vmstate_pmac = {
>>      .name = "ide",
>> -    .version_id = 3,
>> +    .version_id = 4,
>>      .minimum_version_id = 0,
>>      .fields = (VMStateField[]) {
>>          VMSTATE_IDE_BUS(bus, MACIOIDEState),
>>          VMSTATE_IDE_DRIVES(bus.ifs, MACIOIDEState),
>> +        VMSTATE_BOOL(dma_active, MACIOIDEState),
>>          VMSTATE_END_OF_LIST()
>>      }
>>  };
>>
> 
> Did you wind up ever observing this value to be non-zero when it was
> written to the migration stream?
> 
> I really did think that we should be able to assume this was always
> false due to how migration will drain all outstanding AIO, but maybe I
> am mistaken.

I think this can happen because Darwin/MacOS sets the DBDMA processor
running first *before* the IDE request is issued, compared to pretty
much every other OS which issues the IDE request *first* which then in
turn invokes the DMA engine (which is the general assumption in the QEMU
IDE/DMA APIs).

So there could be a window where the DBDMA is programmed and active but
migration takes place before the corresponding IDE request has been
issued (which is exactly the situation that this flag handles).


ATB,

Mark.