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Date: Thu, 26 Nov 2009 15:21:42 +0100
From: Gerd Hoffmann <kraxel@redhat.com>
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Subject: Re: [Qemu-devel] [sneak preview] major scsi overhaul
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To: Hannes Reinecke <hare@suse.de>
Cc: Paul Brook <paul@codesourcery.com>, qemu-devel@nongnu.org

On 11/26/09 12:20, Hannes Reinecke wrote:
> Gerd Hoffmann wrote:
>> /me looks at drivers/scsi/sd.c:sd_done()
>>
>> I can't see any sane way to tell linux that the request was too big.
>>
> residuals is the key:
>
> drivers/scsi/scsi.c:scsi_finish_command()
>
>
> 	good_bytes = scsi_bufflen(cmd);
>          if (cmd->request->cmd_type != REQ_TYPE_BLOCK_PC) {
> 		int old_good_bytes = good_bytes;
> 		drv = scsi_cmd_to_driver(cmd);
> 		if (drv->done)
> 			good_bytes = drv->done(cmd);

drv->done() actually is sd_done() mentioned above.

sd_done() tries to figure how many sectors it actually got for serious 
errors.  I don't feel signaling "medium error" for the first sector 
behind our limit just because we'd like to have smaller requests.

> 		/*
> 		 * USB may not give sense identifying bad sector and
> 		 * simply return a residue instead, so subtract off the
> 		 * residue if drv->done() error processing indicates no
> 		 * change to the completion length.
> 		 */
> 		if (good_bytes == old_good_bytes)
> 			good_bytes -= scsi_get_resid(cmd);

Poor mans bad sector identification.  Same issue as above IMHO.  On top 
of that I wouldn't expect all other guest OSes having the same quirk in 
there to handle usb disks.

cheers,
   Gerd