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Re: [Qemu-devel] [PATCH 7/8] pseries: savevm support for PAPR virtual SC

From: Paolo Bonzini
Subject: Re: [Qemu-devel] [PATCH 7/8] pseries: savevm support for PAPR virtual SCSI
Date: Fri, 31 May 2013 12:41:31 +0200
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:17.0) Gecko/20130514 Thunderbird/17.0.6

Il 31/05/2013 12:25, Alexey Kardashevskiy ha scritto:
> On 05/31/2013 08:07 PM, Benjamin Herrenschmidt wrote:
>> On Fri, 2013-05-31 at 15:58 +1000, Alexey Kardashevskiy wrote:
>>> And another question (sorry I am not very familiar with terminology but
>>> cc:Ben is :) ) - what happens with indirect requests if migration happened
>>> in the middle of handling such a request? virtio-scsi does not seem to
>>> handle this situation anyhow, it just reconstructs the whole request and
>>> that's it.
>> So Paolo, the crux of the question here is really whether we have any
>> guarantee about the state of the request when this happens (by this I
>> mean a save happening with requests still "in flight") ?
>> IE. Can the request can be at any stage of processing, with the data
>> transfer phase being half way through, or do we somewhat know for sure
>> that the request will *not* have started transferring any data ?
>> This is key, because in the latter case, all we really need to do is
>> save the request itself, and re-parse it on restore as if it was
>> new really (at least from a DMA descriptor perspective).
>> However, if the data transfer is already half way through, we need to
>> somewhat save the state of the data transfer machinery, ie. the position
>> of the "cursor" that follows the guest-provided DMA descriptor list,
>> etc... (which isn't *that* trivial since we have a concept of indirect
>> descriptors and we use pointers to follow them, so we'd probably have
>> to re-walk the whole user descriptors list until we reach the same position).

It may be halfway through, but it is always restarted on the destination.

virtio-scsi parses the whole descriptor chain upfront and sends the
guest addresses in the migration stream.

> Is not it the same QEMU thread which handles hcalls and QEMU console
> commands so the migration cannot stop parsing/handling a vscsi_req?

The VM is paused and I/O is flushed at the point when the reqs are sent.
 That's why you couldn't get a pending request.  Only failed requests
remain in queue.


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