[Xen-devel] Re: xl create should refuse to share block devices RW between domains

[Ian Jackson <Ian.Jackson@xxxxxxxxxxxxx>]

Jeremy Fitzhardinge writes ("Re: xl create should refuse to share block devices 
RW between domains"):
> Well, my specific use case is that I have pairs of domain configs, one 
> PV, one HVM, referring to the same set of resources.  I want xl create 
> to catch when I try and create the PV version of a domain while the HVM 
> is still running.

Mmm.  Of course an HVM domain needs to open the underlying device
twice, once for blktap and once for qemu.

> A more comprehensive check would be nice, but just this would be 
> useful.  But whatever it does check should be 100% reliable.

Well, I guess I meant:

 1. Do we have to catch every possible conflict ?  If so then
    your e2fsprogs example is one we need to consider, and we
    will have to add a new kernel feature which can prevent e2fsprogs
    from opening the block device, or simulate "mounting" it or
    something.

 1b. If not, then which conflicts are we trying to detect ?

 2. If we catch a particular combination (eg, start two domains at
    once using the same storage resources) does our check have to be
    race-free ?  That may make it more complicated - and if the answer
    to my first question is "no" there will be some things which are
    inherently racy (eg, spotting mounting a domain's disk
    vs. starting a domain with a disk which is mounted).

NB that when we fix the bug that you can start multiple domains with
the same name, you'll be able to avoid your PV/HVM accident by
specifying the same name in each config file.

Ian.

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