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

[Jan Kiszka <jan.kiszka@web.de>]

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On 2011-05-18 22:02, Alex Williamson wrote:
> On Wed, 2011-05-18 at 14:34 -0500, Anthony Liguori wrote:
>> On 05/18/2011 02:27 PM, Jan Kiszka wrote:
>>> On 2011-05-18 21:10, Anthony Liguori wrote:
>>>> On 05/18/2011 10:30 AM, Jan Kiszka wrote:
>>>> You really don't need to register 90% of the time.  In the case of a PC
>>>> with i440fx, it's really quite simple:
>>>>
>>>> if an I/O is to the APIC page,
>>>>     it's handled by the APIC
>>>
>>> That's not that simple. We need to tell apart:
>>>   - if a cpu issued the request, and which one =>  forward to APIC
>>
>> Right, but what I'm saying is that this logic lives in 
>> kvm-all.c:kvm_run():case EXIT_MMIO.
>>
>> Obviously for TCG, it's a bit more complicated but this should be 
>> handled way before there's any kind of general dispatch.
>>
>>>   - if the range was addressed by a device (PCI or other system bus
>>>     devices) =>  forward to MSI or other MMIO handlers
>>
>> The same is true for MSI.  Other MMIO handlers can be handled as 
>> appropriate.  For instance, once an I/O is sent to the PCI bus, you can 
>> walk each PCI device's BAR list to figure out which device owns the I/O 
>> event.
>>
>> For ISA, it's a little trickier since ISA doesn't do positive decoding. 
>>   You need each ISA device to declare what I/O addresses it handles.
> 
> Do we only need to handle CPU based I/O with this API?  I would think we
> would be layering memory regions and implementing them as a hierarchy
> that reflects the actual hardware layout we're emulating.  An access
> from an I/O device may get a different translation to memory than a CPU
> (IOMMU).  We also might have a system with two VGA devices that both
> register 0xa0000 with a switch in the chipset that decides which one
> sees the accesses, just as real hardware does. ISTR a presentation at
> one of the first KVM forums from you that talked about this type of
> model.  Thanks,

IIUC, that switch is present in every PCI bridge. It can forward legacy
VGA I/O request to its devices or ignore them.

Jan


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