Re: RFC: vfio interface for platform devices

[Scott Wood <scottwood@freescale.com>]

On 07/02/2013 08:07:53 PM, Alexander Graf wrote:
> 
> On 03.07.2013, at 01:25, Yoder Stuart-B08248 wrote:
> 
> > 8.  Open Issues
> >
> >   -how to handle cases where VFIO is requested to handle
> >    a device where the valid, mappable range for a region
> >    is less than a page size.   See example above where an
> >    advertised region in the DMA node is 4 bytes.  If exposed
> >    to a guest VM, the guest has to be able to map a full page
> >    of I/O space which opens a potential security issue.
> 
> The way we solved this for legacy PCI device assignment was by going  
> through QEMU for emulation and falling back to legacy read/write  
> IIRC. We could probably do the same here. IIRC there was a way for a  
> normal Linux mmap'ed device region to trap individual accesses too,  
> so we could just use that one too.
> 
> The slow path emulation would then happen magically in QEMU, since  
> MMIO writes will get reinjected into the normal QEMU MMIO handling  
> path which will just issue a read/write on the mmap'ed region if it's  
> not declared as emulated.

I agree that's what should happen by default, but there should be a way  
for root to tell vfio that a device is allowed to overmap, in order to  
get the performance benefit of direct access in cases where root knows  
(or explicitly doesn't care) that it is safe.

-Scott