Re: [PATCH] On unmap, flush IOMMU TLB and return correct size

[Alex Williamson <alex.williamson-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>]

On Wed, 2013-09-25 at 17:54 +0200, Joerg Roedel wrote:
> On Sat, Sep 21, 2013 at 09:59:00PM -0500, David Woodhouse wrote:
> > I *hate* the bizarre calling convention for iommu_unmap(). Is it
> > actually clearly documented anywhere? Why on earth is it not just
> > returning void, and expected to unmap what it was *asked* to unmap?
> 
> Yeah, I agree that this should be documented since it is quite
> non-standard/non-obvious behaviour of a function. The reason the
> interface was implemented this way is that the caller should not need to
> know (or keep track of) the page-size which was used to map a given iova.
> 
> So the interface is basically, that you give an iova and a size and the
> iommu driver unmaps _at_least_ a region of that size. But if you ask for
> a 4k region which is mapped by a 2M page then the whole 2M are unmapped.
> The return value tells you how much was actually unmapped.
> 
> Not the best interface, I know. We should come up with a better way to
> handle this.

Actually, unmap can return zero too, which happens if you try to unmap
an offset into a super page mapping.  The current interface has issues,
but both legacy kvm assignment and vfio rely on it so we don't need to
track individual mappings.  Thanks,

Alex