Re: [Qemu-devel] [PATCH 0/15][RFC] New PCI interfaces

[Anthony Liguori <anthony@codemonkey.ws>]

On 02/10/2010 12:34 PM, Blue Swirl wrote:
> On Wed, Feb 10, 2010 at 12:01 AM, Anthony Liguori<aliguori@us.ibm.com>  wrote:
>    
>> This is a work in progress that I wanted to share giving some of the discussions
>> around rwhandlers.  The idea is to make PCI devices have a common set of
>> functions to interact with the CPU that is driven entirely through the PCI bus.
>>
>> I've tested the network card conversions, but have not yet tested the other
>> bits.
>>      
> By itself, the patches look OK. But given that the conclusion of the
> generic DMA discussion was to aim for mapping approach (in order to
> prepare for zero copy DMA), I wonder how the patches fit to the larger
> picture. Perhaps instead of pci specific functions, a more generic
> memory object with read and write methods could be introduced, which
> would be more useful in the longer term?
>    

This is a good point and it's something that needs to be addressed for 
the virtio conversion.

I was thinking:

void *pci_memory_map(PCIDevice *dev, pcibus_t addr, pcibus_t *plen, int 
is_write);

void pci_memory_unmap(PCIDevice *dev, void *buf, pcibus_t *plen, int 
is_write, pcibus_t access_len);

Not too surprising.

Since virtio devices can live on two busses (sysbus with Syborg or PCI), 
we need to introduce a set of virtio specific functions.

void virtio_memory_read(VirtIODevice *dev, uint64_t addr, void *buf, int len);
void virtio_memory_write(VirtIODevice *dev, uint64_t addr, const void *buf, int len);
void *virtio_memory_map(VirtIODevice *dev, uint64_t addr, uint64_t *plen, int is_write);
void virtio_memory_unmap(VirtIODevice *dev, uint64_t addr, uint64_t *plen, int is_write, uint64_t access_len);

Inside the VirtIODevice, there would be corresponding function pointers, and depending on whether it was a PCI device or a Syborg device, it would call pci_memory_map or cpu_physical_memory_map.

If we introduced a consistent address type, it would be possible to make generic memory access functions that used a state variable to mask this from the user.

I personally prefer the explicit interfaces though.  It makes it easy to look at a PCI device and see that it only uses PCI interfaces.  I also think that some bus concepts will be difficult to abstract in a generic way.

Regards,

Anthony Liguori