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

[Anthony Liguori <anthony@codemonkey.ws>]

On 05/20/2011 03:56 AM, Avi Kivity wrote:
> On 05/19/2011 07:36 PM, Anthony Liguori wrote:
>>> 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.
>>
>
> It doesn't follow.
>
>> Why do we need priorities at all? There should be no overlap at each
>> level in the hierarchy.
>
> Of course there is overlap. PCI BARs overlap each other, the VGA windows
> and ROM overlap RAM.

Here's what I'm still struggling with:

If children normally overlap their parents, but child priorities are 
always less than their parents, then what's the benefit of having 
anything more than two priorities settings.

As far as I can understand it, a priority of 0 means "let children 
windows overlap" whereas a priority of 1 means "don't let children 
windows overlap".

Is there a use-case for a priority above 1 and if so, what does it mean?

>> 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.
>
> BARs in general don't need priority, except we need to decide if BARs
> overlap RAM of vice-versa.
>
>>
>> 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.
>
> That's what *exactl* what priority means. Which device is in front, and
> which is in the back.

Why not use registration order to resolve this type of conflict?  What 
are the use cases to use priorities where registration order wouldn't be 
adequate?

>> There is no need to have centralized logic to decide this.
>>
>
> I think you're completely missing the point of my proposal.

I'm struggling to find the mental model for priorities.  I may just be 
dense here but the analogy of transparent window ordering isn't helping me.

Regards,

Anthony Liguori