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

[Tian, Kevin]

> From: Nicolin Chen <nicolinc@xxxxxxxxxx>
> Sent: Friday, July 28, 2023 12:43 PM
> 
> On Fri, Jul 28, 2023 at 03:45:39AM +0000, Tian, Kevin wrote:
> > > 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.
> 
> I can understand that the user should reconfigure the IOAS on
> success. Yet, should we expect it to reconfigure on a failure
> also?
> 

I thought the user will likely stop the device before changing IOAS
and then re-enable device DMA afterwards. If that is the typical
flow then no matter this replace request succeeds or fails the
re-enabling sequence should lead to the addition of pinned pages
back to the current IOAS.

But this does imply inconsistent behavior between success and failure.
Not sure whether it's worth a fix e.g. introducing another notifier for
mdev drivers to re-pin...