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Date: Thu, 29 Oct 2009 05:37:08 +0100
From: Christoph Hellwig <hch@lst.de>
Subject: Re: [Qemu-devel] [PATCH 0/4] megaraid_sas HBA emulation
Message-ID: <20091029043708.GA3552@lst.de>
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To: Hannes Reinecke <hare@suse.de>
Cc: virtualization@lists.linux-foundation.org, Gerd Hoffmann <kraxel@redhat.com>, kvm@vger.kernel.org, qemu-devel@nongnu.org

On Wed, Oct 28, 2009 at 08:25:22PM +0100, Hannes Reinecke wrote:
> I don't think we really need two modes.
> My preferred interface here is to pass down scatter-gather lists down
> with every xfer; this way it'll be the responsibility of the driver to
> create the lists in the first place. If it has hardware scatter-gather
> support it can just pass them down, if not it can as easily create a
> scatter-gather list with just one element as a bounce buffer.

Yes.  If this really causes performance problems for esp we can add
bounce buffering in that driver later.

> So something like
> - Get next request
> - Attach iovec/bounc-buffer
> - handle request (command/write/read)
> - complete request (callback)

Btw, from some previuous attempts to sort out this code here are some
thing that I think would be beneficial:

 - try to create generic scsi device/request structures that the hba
   driver can access, and which contain additional private data for
   scsi-disk/generic.  Information in the generic one would include
   the information about the data transfer, the tag and the command
   block.
 - try to get rid of the tag indexing stuff by just using a pointer to
   the generic scsi request in the hba drivers.  That should get rid
   of a lot of useless list searching.