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Re: [Qemu-devel] [sneak preview] major scsi overhaul
From: |
Paul Brook |
Subject: |
Re: [Qemu-devel] [sneak preview] major scsi overhaul |
Date: |
Wed, 11 Nov 2009 04:06:31 +0000 |
User-agent: |
KMail/1.12.2 (Linux/2.6.30-2-amd64; KDE/4.3.2; x86_64; ; ) |
On Friday 06 November 2009, Gerd Hoffmann wrote:
> Hi,
>
> http://repo.or.cz/w/qemu/kraxel.git/shortlog/refs/heads/scsi.v6
>
> What is in there?
> (3) New interface for HBA <=> SCSIDevice interaction:
> * building on the new SCSIRequest struct.
> * the silly read_data/write_data ping-pong game is gone.
> * support for scatter lists and zerocopy I/O is there.
The "silly ping-pong game" is there for a reason.
Your code assumes that the whole transfer will be completed in a single
request. This is not true for any of the current HBAs. Expecting the HBA to
cache the entire response is simply not acceptable. Remember that a single
transfer can be very large (greater than available ram on the host). Even with
a SG capable HBA, you can not assume all the data will be transferred in a
single request.
You should also consider how this interacts with command queueing.
IIUC an Initiator (HBA) typically sends a tagged command, then disconnects
from the target (disk). At some later time the target reconnects, and the
initiator starts the DMA transfer. By my reading your code does not issue any
IO requests until after the HBA starts transferring data. The only way to
achieve this is for the target to pretend it has data available immediately,
at which point the transfer stalls and we loose the opportunity for parallel
command queueing.
On a cosmetic level, you seem somewhat schizophrenic between _req_ and
_request_.
Paul
- [Qemu-devel] [sneak preview] major scsi overhaul, Gerd Hoffmann, 2009/11/06
- Re: [Qemu-devel] [sneak preview] major scsi overhaul, Blue Swirl, 2009/11/07
- Re: [Qemu-devel] [sneak preview] major scsi overhaul, Gerd Hoffmann, 2009/11/09
- Re: [Qemu-devel] [sneak preview] major scsi overhaul, Avi Kivity, 2009/11/09
- Re: [Qemu-devel] [sneak preview] major scsi overhaul, Gerd Hoffmann, 2009/11/09
- Re: [Qemu-devel] [sneak preview] major scsi overhaul, Avi Kivity, 2009/11/09
- Re: [Qemu-devel] [sneak preview] major scsi overhaul, Gerd Hoffmann, 2009/11/09
- Re: [Qemu-devel] [sneak preview] major scsi overhaul, Avi Kivity, 2009/11/09
- Re: [Qemu-devel] [sneak preview] major scsi overhaul, Blue Swirl, 2009/11/09
- Re: [Qemu-devel] [sneak preview] major scsi overhaul, Gerd Hoffmann, 2009/11/09
Re: [Qemu-devel] [sneak preview] major scsi overhaul,
Paul Brook <=
- Re: [Qemu-devel] [sneak preview] major scsi overhaul, Gerd Hoffmann, 2009/11/11
- Re: [Qemu-devel] [sneak preview] major scsi overhaul, Paul Brook, 2009/11/11
- Re: [Qemu-devel] [sneak preview] major scsi overhaul, Gerd Hoffmann, 2009/11/11
- Re: [Qemu-devel] [sneak preview] major scsi overhaul, Paul Brook, 2009/11/11
- Re: [Qemu-devel] [sneak preview] major scsi overhaul, Gerd Hoffmann, 2009/11/16
- Re: [Qemu-devel] [sneak preview] major scsi overhaul, Paul Brook, 2009/11/16
- Re: [Qemu-devel] [sneak preview] major scsi overhaul, Gerd Hoffmann, 2009/11/16
- Re: [Qemu-devel] [sneak preview] major scsi overhaul, Gerd Hoffmann, 2009/11/24
- Re: [Qemu-devel] [sneak preview] major scsi overhaul, Paul Brook, 2009/11/24
- Re: [Qemu-devel] [sneak preview] major scsi overhaul, Gerd Hoffmann, 2009/11/25