RE: [PATCH v8 2/4] iommufd: Add iommufd_access_replace() API

[Tian, Kevin]

> From: Nicolin Chen <nicolinc@xxxxxxxxxx>
> Sent: Friday, July 28, 2023 3:04 AM
> 
> On Thu, Jul 27, 2023 at 09:03:01AM -0300, Jason Gunthorpe wrote:
> > On Wed, Jul 26, 2023 at 07:59:11PM -0700, Nicolin Chen wrote:
> >
> > > I just realized that either my v8 or your version calls unmap()
> > > first at the entire cur_ioas. So, there seems to be no point in
> > > doing that fallback re-add routine since the cur_ioas isn't the
> > > same, which I don't feel quite right...
> >
> > The point is to restore the access back to how it should be on failure
> > so future use of the accesss still does the right thing.
> >
> > We already have built into this a certain non-atomicity for mdevs,
> > they can see a pin failure during replace if they race an access
> > during this unmap window. This is similar to the real HW iommu's
> > without atomic replace.
> 
> I was concerned about, after the replace, mdev losing all the
> mappings due to the unmap() call, which means the fallback is
> not really a status quo. Do you mean that they could pin those
> lost mappings back?

None of mdev drivers does that.

but we need think about the actual usage. I don't think the user
can request ioas change w/o actually reconfiguring the mdev
device. Presumably the latter could lead to reconstructure of pinned
pages.

so in code-level as Jason said we just need ensure the access is
back to an usable state.