Re: [libvirt] mdevctl: A shoestring mediated device management and persistence utility

[Cornelia Huck <cohuck@redhat.com>]

On Fri, 14 Jun 2019 10:04:18 -0600
Alex Williamson <alex.williamson@redhat.com> wrote:

> On Fri, 14 Jun 2019 17:06:15 +0200
> Christophe de Dinechin <cdupontd@redhat.com> wrote:
> 
> > > On 14 Jun 2019, at 16:23, Alex Williamson <alex.williamson@redhat.com> wrote:
> > > 
> > > On Fri, 14 Jun 2019 11:54:42 +0200
> > > Christophe de Dinechin <cdupontd@redhat.com> wrote:

> > > Where is the parent/type ownership implied?    
> > 
> > I did not imply it, but I read some concern about ownership
> > on your part in "they need to guess that an mdev device
> > with the same parent and type is *theirs*.” (emphasis mine)
> > 
> > I personally see no change on the “need to guess” implied
> > by the fact that you run uuidgen inside the script, so
> > that’s why I tried to guess what you meant.  
> 
> As I noted in the reply to the pull request, putting `uuidgen` inline
> was probably a bad example. 

FWIW, I just sent a pull req to get rid of that inline `uuidgen` in the
example.

> However, the difference is that the user
> has imposed the race on themselves if they invoke mdevctl like this,
> they've provided a uuid but they didn't record what it is.  This is the
> user's problem.  Pushing uuid selection into mdevctl makes it mdevctl's
> problem because the interface is fundamentally broken.
> 
> > > The intended semantics are
> > > "try to create this type of device under this parent”.    
> > 
> > Agreed. Which is why I don’t see why trying to create
> > with some new UUID introduces any race (as long as
> > the script prints out that UUID, which I admit my patch
> > entirely failed to to)  
> 
> And that's the piece that makes it fundamentally broken.  Beyond that,
> it seems unnecessary.  I don't see this as the primary invocation of
> mdevctl and the functionality it adds is trivially accomplished in a
> wrapper, so what's the value?
> 
> > >>> How do you resolve two instances of this happening in parallel and both
> > >>> coming to the same conclusion which is their device.  If a user wants
> > >>> this sort of headache they can call mdevctl with `uuidgen` but I don't
> > >>> think we should encourage it further.      
> > >> 
> > >> I agree there is a race, but if anything, having a usage where you don’t
> > >> pass the UUID on the command line is a step in the right direction.
> > >> It leaves the door open for the create-mdev script to do smarter things,
> > >> like deferring the allocation of the mdevs to an entity that has slightly
> > >> more knowledge of the global system state than uuidgen.    
> > > 
> > > A user might (likely) require a specific uuid to match their VM
> > > configuration.  I can only think of very niche use cases where a user
> > > doesn't care what uuid they get.    
> > 
> > They do care. But I typically copy-paste my UUIDs, and then
> > 
> > 1. copy-pasting at the end is always faster than between
> > the command and other arguments (3-args case). 
> > 
> > 2. copy-pasting the output of the previous command is faster
> > than having one extra step where I need to copy the same thing twice
> > (2-args case).
> > 
> > So to me, if the script is intended to be used by humans, my
> > proposal makes it slightly more comfortable to use. Nothing more.  
> 
> This is your preference, but I wouldn't call it universal.  Specifying
> the uuid last seems backwards to me, we're creating an object so let's
> first name that object.  We then specify where that object should be
> created and what type it has.  This seems very logical to me, besides,
> it's also the exact same order we use when listing mdevs :P
> 
> Clearly there's personal preference here, so let's not arbitrarily pick
> a different preference.  If copy/paste order is more important to you
> then submit a patch to give mdevctl real argument processing so you can
> specify --uuid, --parent, --type in whatever order you want.

I agree that these are personal preferences :) Real argument processing
makes sense, however.