Re: [Qemu-devel] [RFC] Memory API

[Gleb Natapov]

On Thu, May 19, 2011 at 06:30:42PM +0200, Jan Kiszka wrote:
> On 2011-05-19 18:28, Gleb Natapov wrote:
> > On Thu, May 19, 2011 at 06:25:14PM +0200, Jan Kiszka wrote:
> >> On 2011-05-19 18:17, Gleb Natapov wrote:
> >>> On Thu, May 19, 2011 at 05:40:50PM +0300, Avi Kivity wrote:
> >>>> On 05/19/2011 05:37 PM, Anthony Liguori wrote:
> >>>>>
> >>>>> So....  do you do:
> >>>>>
> >>>>> isa_register_region(ISABus *bus, MemoryRegion *mr, int priority)
> >>>>> {
> >>>>>    chipset_register_region(bus->chipset, mr, priority + 1);
> >>>>> }
> >>>>>
> >>>>> I don't really understand how you can fold everything into one
> >>>>> table and not allow devices to override their parents using
> >>>>> priorities.
> >>>>
> >>>> Think of how a window manager folds windows with priorities onto a
> >>>> flat framebuffer.
> >>>>
> >>>> You do a depth-first walk of the tree.  For each child list, you
> >>>> iterate it from the lowest to highest priority, allowing later
> >>>> subregions override earlier subregions.
> >>>>
> >>> And how you set those priorities in a sensible way? Why two device on a
> >>> PCI bus will want to register their memory region with anything but
> >>> highest priority? And if you let PCI subsystem to assign priorities how
> >>> it will coordinate with ISA subsystem/memory controller what priorities
> >>> to assign to get meaningful system?
> >>
> >> Priorities >default will only be used for explicit overlays, e.g. RAM
> >> over MMIO in PAM regions. Non-default priorities won't be assigned to
> >> normal PCI bars or any other device's region.
> >>
> > That does not explain who and how assign those priorities in globally
> > meaningful way.
> 
> There are no global priorities. Priorities are only used inside each
> level of the memory region hierarchy to generate a resulting, flattened
> view for the next higher level. At that level, everything imported from
> below has the default prio again, ie. the lowest one.
> 
Ah, so you are advocating for filtering on each level then. Because
highest level (the one that actually uses memory API) will never see
two regions with different priorities since layer bellow will flatten
the memory layout. So why do you need priorities if this is the case?
The layer that does flattening is the layer that assign priorities
anyway.

--
                        Gleb.