On Mon, Oct 17, 2011 at 12:34:19PM +0200, Avi Kivity wrote:
> On 10/17/2011 07:31 AM, David Gibson wrote:
> > >
> > > In terms of how the code looks, it's seriously more ugly (see the
> > > patches I sent out). Conceptually it's cleaner, since we're not dodging
> > > the issue that we need to deal with a full 64-bit domain.
> >
> > We don't have to dodge that issue. I know how to remove the
> > requirement for intermediate negative values, I just haven't made up a
> > patch yet. With that we can change to uint64 and cover the full 64
> > bit range. In fact I think I can make it so that size==0 represents
> > size=2^64 and even handle the full 64-bit, inclusive range properly.
>
> That means you can't do a real size == 0.
Yeah... a memory range with size 0 has no effect by definition, I
think we can do without it.
> > > But my main concern is maintainability. The 64-bit blanket is to short,
> > > if we keep pulling it in various directions we'll just expose ourselves
> > > in new ways.
> >
> > Nonsense, dealing with full X-bit range calculations in X-bit types is
> > a fairly standard problem. The kernel does it in VMA handling for
> > one. It just requires thinking about overflow cases.
>
> We discovered three bugs already (you found two, and I had one during
> development). Even if it can probably be done with extreme care, but is
> it worth spending all that development time on?
>
> I'm not sure there is a parallel with vmas, since we're offsetting in
> both the positive and negative directions.
I think the so-called "negative offsetting" is just an artifact of our
implementation. I don't see that it's any different from having a VMA
whose file offset is larger than its memory address.
--
David Gibson | I'll have my music baroque, and my code
david AT gibson.dropbear.id.au | minimalist, thank you. NOT _the_ _other_
| _way_ _around_!
http://www.ozlabs.org/~dgibson
