Re: [Qemu-devel] [PATCH 1/9] Add stub functions for PCI device models to do PCI DMA

[Anthony Liguori <aliguori@us.ibm.com>]

On 10/02/2011 05:25 AM, Michael S. Tsirkin wrote:
> On Mon, Sep 05, 2011 at 02:34:56PM +1000, 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>
>
> So something I just thought about:
>
> all wrappers now go through cpu_physical_memory_rw.
> This is a problem as e.g. virtio assumes that
> accesses such as stw are atomic. cpu_physical_memory_rw
> is a memcpy which makes no such guarantees.

That's why I asked for David to add map/unmap functions to the API.

Regards,

Anthony Liguori