Re: [Qemu-devel] [PATCH 01/10] Add stub functions for PCI device models to do PCI DMA

[Anthony Liguori <anthony@codemonkey.ws>]

On 09/01/2011 11:03 AM, Avi Kivity wrote:
> On 09/01/2011 06:55 PM, Anthony Liguori wrote:
>> On 09/01/2011 12:00 AM, David Gibson wrote:
>>> This patch adds functions to pci.[ch] to perform PCI DMA operations. At
>>> present, these are just stubs which perform directly cpu physical memory
>>> accesses.
>>>
>>> Using these stubs, however, distinguishes PCI device DMA transactions
>>> from
>>> other accesses to physical memory, which will allow PCI IOMMU support to
>>> be added in one place, rather than updating every PCI driver at that
>>> time.
>>>
>>> That is, it allows us to update individual PCI drivers to support an
>>> IOMMU
>>> without having yet determined the details of how the IOMMU emulation
>>> will
>>> operate. This will let us remove the most bitrot-sensitive part of an
>>> IOMMU patch in advance.
>>>
>>> Signed-off-by: David Gibson<david@gibson.dropbear.id.au>
>>
>> I think this is the wrong approach given the introduction of the
>> memory API.
>>
>> I think we should have a generic memory access function that takes a
>> MemoryRegion as it's first argument.
>>
>> The PCI bus should then expose one memory region for each device
>> (that's how it can figure out where the access is coming from).
>>
>> Richard/Avi, what do you think?
>>
>
> I think the patchset is fine. It routes all access through pci_dma_rw(),
> which accepts a PCIDevice. We can later define pci_dma_rw() in terms of
> the memory API and get the benefit of the memory hierarchy.

The challenge is what you do about something like ne2k where the core 
chipset can either be a PCI device or an ISA device.  You would have to 
implement a wrapper around pci_dma_rw() in order to turn it into 
cpu_physical_memory_rw when doing ISA.

Regards,

Anthony Liguori

>