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

[Jan Kiszka <jan.kiszka@siemens.com>]

On 2011-05-19 15:44, Anthony Liguori wrote:
> On 05/19/2011 04:10 AM, Avi Kivity wrote:
>> On 05/19/2011 12:08 PM, Gleb Natapov wrote:
>>> On Wed, May 18, 2011 at 06:42:14PM +0300, Avi Kivity wrote:
>>>> On 05/18/2011 06:36 PM, Jan Kiszka wrote:
>>>>>>
>>>>>> We need to head for the more hardware-like approach. What happens
>>> when
>>>>>> you program overlapping BARs? I imagine the result is
>>>>>> implementation-defined, but ends up with one region decoded in
>>>>>> preference to the other. There is simply no way to reject an
>>>>>> overlapping mapping.
>>>>>
>>>>> But there is also now simple way to allow them. At least not without
>>>>> exposing control about their ordering AND allowing to hook up managing
>>>>> code (e.g. of the PCI bridge or the chipset) that controls
>>> registrations.
>>>>
>>>> What about memory_region_add_subregion(..., int priority) as I
>>>> suggested in another message?
>>> Haven't saw another message yet, but how caller knows about priority?
>>
>> The caller is emulating some hub or router and should decide on priority
>> like real hardware.
>>
>> For example, piix gives higher priority to the vga window over RAM.
> 
> Well......
> 
> The i440fx may direct VGA accesses to RAM depending on the SMM 
> registers.  By the time the PIIX gets the I/O request, we're past the 
> memory controller.
> 
> This is my biggest concern about this whole notion of "priority".  These 
> sort of issues are not dealt with by a simple z-ordering.  There is 
> logic in each component that may be arbitrarily complex.
> 
> We're going to end up having to dynamically change the "priority" based 
> how registers are programmed.  But priorities are relative so it's 
> unclear to me how the I440FX would prioritize RAM over dispatch to PIIX 
> (for VGA, for instance).

But creating an extra RAM window region with higher priority than the
underlying mappings.

Jan

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