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

[Gleb Natapov <gleb@redhat.com>]

On Thu, May 19, 2011 at 06:49:48PM +0200, Jan Kiszka wrote:
> On 2011-05-19 18:36, Anthony Liguori wrote:
> > On 05/19/2011 11:30 AM, 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.
> > 
> > Then SMM is impossible.
> 
> For sure it is. The CPU and the chipset, each at their mapping level,
> create a corresponding RAM region and register it with higher prio at
> the SMRAM start address (CPU and chipset will need to exchange that
> address or otherwise coordinate the mapping information - the price for
> per-CPU SMRAM).
> 
So to get priorities right two components need to know a priori about
overlap and coordinate the priorities?

> > 
> > Why do we need priorities at all?  There should be no overlap at each 
> > level in the hierarchy.
> > 
> > If you have overlapping BARs, the PCI bus will always send the request 
> > to a single device based on something that's implementation specific. 
> > This works because each PCI device advertises the BAR locations and 
> > sizes in it's config space.
> 
> That's not a use case for priorities at all. Priorities are useful for
> PAM and SMRAM-like scenarios.
> 
It looks like you are talking about very shallow model were overlap may
happen only in chipset and chipset directly controls all of the physical
address space. But we need to have solution for more complex topologies
where PAM/SMRAM like scenarios may happen on each level. You are dismissing
PCI as an example because all memory regions there are of the same
priority, but this is just the special case of more generic scenario.
Why this is not the "use case for priorities"?

--
			Gleb.