[libvirt] [PATCH] qemu: Fix detection of drive readonly option

[Daniel Veillard]

On Tue, Oct 26, 2010 at 10:36:15AM +0100, Daniel P. Berrange wrote:
> On Mon, Oct 25, 2010 at 03:56:58PM -0600, Eric Blake wrote:
> > It feels like we're burning through these flag bits rather quickly: 15 
> > flags added since January of this year.  While it doesn't affect your 
> > patch, it means that at the current pace we have less than 18 months 
> > before we have to add a second flag variable.
> 
> This isn't a problem. This is an internal ABI only, so we can change the
> way we represent this anytime we like.
> 
> There is a bigger problem in this area. We run 'qemu -help' every time
> we do any operation on the guest that requires these flags to be known.
> This is bad because the 'qemu' binary may not be the same qemu binary
> that we originally booted the guest with (ie RPM package upgrade). We
> can thus detect the wrong flags potentially. 
> 
> What I think we should do is define a bitset type 'virBitset'. Change
> this to a plain enum, instead of a flag based enum, thus giving us
> 2^64 entries. We can then have APIs like  virBitsetIsSet(num). The
> virBitset data should be stored in the virDomainObjPtr instance at
> the time of guest booting. We also need to persist it in the XML state
> file /var/lib/libvirt/qemu/$guest.xml, as named enum entries, and reload
> this at libvirtd restart.
> 
> I'm not likely to have any time to implement this in the forseeable
> future though. Hopefully someone can do it before we use up the last
> 20 flags :-)

  Any chance this could end up in http://libvirt.org/todo.html
in an "Internals" section ?

Daniel

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