Re: [PATCH 0/2] Introduce iommu_commit() function

Avi Kivity
2011-11-16T09:40:29Z

On 06/23/2011 06:38 PM, David Woodhouse wrote:
> On Thu, 2011-06-23 at 17:31 +0200, Joerg Roedel wrote:
> > David, I think especially VT-d can benefit from such a callback. I will
> > implement support for it in the AMD IOMMU driver and post a patch-set
> > soon.
> >
> > Any comments, thoughts?
>
> Ick. We *already* do the flushes as appropriate while we're filling the
> page tables. So every time we move on from one page table page to the
> next, we'll flush the old one. And when we've *done* filling the page
> tables for the range we've been asked to map, we flush the last writes
> too.

For the current kvm use case flushing just once on commit is most
efficient. If/when we get resumable io faults, per-page flushing
becomes worthwhile.

> The problem with KVM is that it calls us over and over again to map a
> single 4KiB page.
>
> It doesn't seem simple to make use of a 'commit' function, because we'd
> have to keep track of *which* page tables are dirty.

You could easily do that by using a free bit in the pte as a dirty bit.
You can then choose whether to use per-page flush or a full flush.

> I'd much rather KVM just gave us a list of the pages to map, in a single
> call.

The list can easily be several million pages long.

> Or even a 'translation' callback we could call to get the physical
> address for each page in the range.

This is doable, and is probably most flexible. If the translation also
returns ranges, then you don't have to figure out large mappings yourself.

Not that there's a huge difference between

iommu_begin(iommu_transaction, domain)
for (page in range)
iommu_map(iommu_transaction, page, translate(page))
iommu_commit(iommu_transaction)

and

iommu_map(domain, range, translate)

- one can be converted to the other.

--
error compiling committee.c: too many arguments to function