> 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...
