qemu-devel
[Top][All Lists]
Advanced

[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

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


From: Anthony Liguori
Subject: Re: [Qemu-devel] [RFC] Memory API
Date: Thu, 19 May 2011 11:36:57 -0500
User-agent: Mozilla/5.0 (X11; U; Linux x86_64; en-US; rv:1.9.2.17) Gecko/20110424 Lightning/1.0b2 Thunderbird/3.1.10

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





reply via email to

[Prev in Thread] Current Thread [Next in Thread]