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Re: [Qemu-devel] [Qemu-block] [PATCH v1 07/15] block: add flag to indica

From: Daniel P. Berrange
Subject: Re: [Qemu-devel] [Qemu-block] [PATCH v1 07/15] block: add flag to indicate that no I/O will be performed
Date: Wed, 13 Jan 2016 17:56:24 +0000
User-agent: Mutt/1.5.24 (2015-08-30)

On Wed, Jan 13, 2016 at 06:44:41PM +0100, Kevin Wolf wrote:
> Am 12.01.2016 um 19:56 hat Daniel P. Berrange geschrieben:
> > When opening an image it is useful to know whether the caller
> > intends to perform I/O on the image or not. In the case of
> > encrypted images this will allow the block driver to avoid
> > having to prompt for decryption keys when we merely want to
> > query header metadata about the image. eg qemu-img info
> > 
> > Signed-off-by: Daniel P. Berrange <address@hidden>
> Would it be worth adding assertions to the block layer I/O functions
> (most importantly, bdrv_aligned_preadv/pwritev) that this flag isn't
> set?

Yes, I think so.

In the read/write code paths that trigger encryption, I'm already doing
an assert that the encryption context is setup, which is indirectly
related to BDRV_O_NO_IO. It is probably worth adding an assertion
explicitly based on the flag at the top level though, as it'd give
a clearer error if it ever got triggered.

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