Re: [PATCH] MMIO: Make coalesced mmio use a device per zone

[Sasha Levin <levinsasha928@gmail.com>]

On Tue, 2011-07-19 at 13:32 +0300, Avi Kivity wrote:
> On 07/19/2011 01:17 PM, Sasha Levin wrote:
> > On Tue, 2011-07-19 at 12:59 +0300, Avi Kivity wrote:
> > >  On 07/19/2011 12:53 PM, Sasha Levin wrote:
> > >  >  >   Make these per-guest instead of global.  The lock may be contended, and
> > >  >  >   the list shouldn't hold items from different guests (why is it needed,
> > >  >  >   anyway?)
> > >  >  >
> > >  >
> > >  >  We only need the list for removal, since we only have the range we want
> > >  >  to remove, and we want to find all devices which contain this range.
> > >  >
> > >
> > >  All devices in the same guest which contain this range.  Your patch
> > >  removes devices from all guests.
> > >
> >
> > Yup. I've messed up guest-locality. Will fix.
> >
> > Also, I found this comment when increasing NR_IOBUS_DEVS:
> >
> > /*
> >   * It would be nice to use something smarter than a linear search, TBD...
> >   * Thankfully we dont expect many devices to register (famous last words :),
> >   * so until then it will suffice.  At least its abstracted so we can change
> >   * in one place.
> >   */
> >
> > Since so far we've registered 5-6 devices, and now it may increase
> > significantly (since we may want to do the same change to ioeventfds,
> > which work the same way) - how would you feel if we make devices
> > register range(s) and do a rbtree lookup instead of a linear search?
> >
> 
> It makes sense.  In fact your change is a good first step - so far it 
> was impossible to to a clever search since the seaching code was not 
> aware of the ranges (and could not be, since the single coalesced mmio 
> device had multiple ranges).
> 
> Rather than an rbtree, I suggest putting all ranges in an array and 
> sorting it, then using binary search.
> 

Why array over rbtree? An array would be heavier on register/unregister,
and using rbtree would let us easily eliminate any max device limit we
had so far.

We've had good experience using interval rbtree in /tools/kvm where we
did PIO and MMIO device mapping using an augmented rbtree which made
range lookups very simple and fast.

-- 

Sasha.