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Re: [Xen-devel] [PATCH]: Make Xen 3.1 IDE flush on O_DIRECT with drive c

To: Chris Lalancette <clalance@xxxxxxxxxx>
Subject: Re: [Xen-devel] [PATCH]: Make Xen 3.1 IDE flush on O_DIRECT with drive caching off
From: "Daniel P. Berrange" <berrange@xxxxxxxxxx>
Date: Tue, 26 Feb 2008 14:51:29 +0000
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On Tue, Feb 26, 2008 at 09:41:30AM -0500, Chris Lalancette wrote:
> All,
>      Long ago Xen added code to the device model to basically do an fsync()
> after every data write if the user in the guest specified that IDE write 
> caching
> should be disabled.  This works fine, except in the case where you are doing
> O_DIRECT writes inside the guest (ala dd if=/dev/zero of=/dev/hdb 
> oflag=direct).
>  This is because you can get out of ide_write_dma_cb() in the middle of the 
> loop
> without going through the logic to sync.  This simple patch makes sure that 
> you
> always check (and sync) inside the write callback.
> This patch applies to xen-3.1-testing.hg; I still have to test 3.2 (and the 
> new
> AIO code) to see if it is affected by this bug.
> Signed-off-by: Chris Lalancette <clalance@xxxxxxxxxx>

  Signed-off-by: Daniel P. Berrange <berrange@xxxxxxxxxx>

I've gone through the logic in ide_write_dma_cb() and  breaking out of the
loop is the thing todo in this scenario. The new AIO code in 3.2 looks like
it suffers from a similar problem, with one of the code paths missing the
sync due to a mis-placed label / goto pair.

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