Re: RFC: vfio API changes needed for powerpc

[Scott Wood <scottwood-KZfg59tc24xl57MIdRCFDg@public.gmane.org>]

On 04/03/2013 02:09:45 PM, Stuart Yoder wrote:
> > Would is be possible for userspace to simply leave room for MSI bank
> > mapping (how much room could be determined by something like
> > VFIO_IOMMU_GET_MSI_BANK_COUNT) then document the API that userspace  
> can
> > DMA_MAP starting at the 0x0 address of the aperture, growing up, and
> > VFIO will map banks on demand at the top of the aperture, growing  
> down?
> > Wouldn't that avoid a lot of issues with userspace needing to know
> > anything about MSI banks (other than count) and coordinating irq  
> numbers
> > and enabling handlers?
> 
> This is basically option #A in the original proposals sent.   I like
> this approach, in that it
> is simpler and keeps user space mostly out of this...which is
> consistent with how things are done
> on x86.  User space just needs to know how many windows to leave at
> the top of the aperture.
> The kernel then has the flexibility to use those windows how it wants.
> 
> But one question, is when should the kernel actually map (and unmap)
> the MSI banks.

I think userspace should explicitly request it.  Userspace still  
wouldn't need to know anything but the count:

count = VFIO_IOMMU_GET_MSI_BANK_COUNT
VFIO_IOMMU_SET_ATTR(ATTR_GEOMETRY)
VFIO_IOMMU_SET_ATTR(ATTR_WINDOWS)
// do other DMA maps now, or later, or not at all, doesn't matter
for (i = 0; i < count; i++)
	VFIO_IOMMU_MAP_MSI_BANK(iova, i);
// The kernel now knows where each bank has been mapped, and can update  
PCI config space appropriately.

> One thing we need to do is enable the aperture...and current
> thinking is that is done on the first DMA_MAP.

What if there are no other mappings required?

-Scott