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

[Paolo Bonzini]

Il 01/06/2013 02:01, Benjamin Herrenschmidt ha scritto:
> On Fri, 2013-05-31 at 12:41 +0200, Paolo Bonzini wrote:
> 
>> It may be halfway through, but it is always restarted on the destination.
> 
> "restarted" as in the whole transfer is restarted if any right ? So we
> can essentially consider as a new request for which we just did
> scsi_req_enqueue() ?
> 
> IE. We don't do direct DMA to guest pages just yet (we still do copies)
> so basically our process is:
> 
>  1- Obtain request from guest
>  2- Queue it (scsi_req_enqueue)
>  3- No transfer -> go away (completion is called)
>  4- Pre-process user descriptors (check desc type direct vs indirect,
>     position our "cursor" walking them etc....)
>  5- scsi_req_continue()
>     .../... loop of callbacks & transfer
> 
> Now from what you say, I assume that regardless of the point where
> the request was, when we "resume" it will always be at step 4 ?
> 
> IE. I can just pre-process the descriptors again ? (I actually need
> to transfer them again from the guest since I suspect I clobber them
> at the very least due to byteswap) and call scsi_req_continue() and
> assume the transfer (if any) started from the beginning ?

Yes.  Unless the spec somehow lets the guest figure out the point at
which the whole chain has been pre-processed, and lets the guest modify
the chain at this point.  But if that's not the case, you can do that.
Memory has already been loaded when load_request runs.

Paolo