Re: [Qemu-devel] [PATCH] memory: use signed arithmetic

[Avi Kivity]

On 08/03/2011 12:59 AM, Richard Henderson wrote:
> On 08/02/2011 01:50 PM, Avi Kivity wrote:
> >   struct AddrRange {
> >  -    uint64_t start;
> >  -    uint64_t size;
> >  +    int64_t start;
> >  +    int64_t size;
>
> I'm must say I'm not keen on this.  My primary objection is that
> a "range" can no longer properly represent the entire address space.
> Or, indeed, anything in the second half of it.

I don't think there's any cpu which has a real 64-bit physical address 
space?  Don't they all truncate it?

> It sounds like your problem would be better solved by re-arranging
> things such that you perform X<  Y comparisons rather than DELTA<  0.


More like, X + delta < Y + delta, I just get a headache what all those 
deltas mean everywhere.

For reference, the root cause is
static void render_memory_region(FlatView *view,
                                 MemoryRegion *mr,
                                 target_phys_addr_t base,
                                 AddrRange clip)
{
...

    base += mr->addr;

...

    if (mr->alias) {
        base -= mr->alias->addr;
        base -= mr->alias_offset;   // <--- HERE!
        render_memory_region(view, mr->alias, base, clip);
        return;
    }

I could pass alias_offset everywhere and compensate for it.  But 
stealing bit from the address space is easier than adjusting all the 
calculations.

--
I have a truly marvellous patch that fixes the bug which this
signature is too narrow to contain.