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

[Anthony Liguori <anthony@codemonkey.ws>]

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.

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.

To dispatch a request, the PCI bus will walk the config space to find a 
match.  If you remove something that was previously causing an overlap, 
it'll the other device will now get the I/O requests.

To model this correctly, you need to let the PCI bus decide how to 
dispatch I/O requests (again, you need hierarchical dispatch).

In the absence of this, the PCI bus needs to look at all of the devices, 
figure out the flat mapping, and register it.  When a device is added or 
removed, it needs to recalculate the flat mapping and register it.

There is no need to have centralized logic to decide this.

Regards,

Anthony Liguori

>
> Jan
>