On 17.11.2021 12:09, Andrew Cooper wrote: > On 17/11/2021 10:13, Jan Beulich wrote: >> On 17.11.2021 09:55, Roger Pau Monné wrote: >>> On Wed, Nov 17, 2021 at 09:39:17AM +0100, Jan Beulich wrote: >>>> On 13.09.2021 11:02, Jan Beulich wrote: >>>>> libxl__domain_config_setdefault() checks whether PoD is going to be >>>>> enabled and fails domain creation if at the same time devices would get >>>>> assigned. Nevertheless setting up of IOMMU page tables is allowed. >>> I'm unsure whether allowing enabling the IOMMU with PoD is the right >>> thing to do, at least for our toolstack. >> May I ask about the reasons of you being unsure? > > PoD and passthrough is a total nonsense. You cannot have IOMMU mappings > to bits of the guest physical address space which don't exist. > > It is now the case that IOMMU (or not) must be specified at domain > creation time, which is ahead of creating PoD pages. Certainly as far > as Xen is concerned, the logic probably wants reversing to have > add_to_physmap&friends reject PoD if an IOMMU was configured. > > A toolstack could, in principle, defer the decision to first device > assignment.
Right, which is what I consider the preferred approach. > However, this is terrible behaviour all around, because one way or > another we've got to force-populate all PoD pages (which is potentially > minutes worth of work to do), Sure. > and liable to suffer -ENOMEM, Not if (as suggested) we first check that the PoD cache is large enough to cover all PoD entries. > or we have > to reject a control operation with -EBUSY for a task which is dependent > on the guest kernel actions in a known-buggy area. Why reject anything? > There is no point trying to make this work. If a user wants a device, > they don't get to have PoD. Anything else is a waste of time and effort > on our behalf for a usecase that doesn't exist in practice. Not sure where you take the latter from. I suppose I'll submit the patch as I have it now (once I have properly resolved dependencies on other patches I have queued and/or pending), and if that's not deemed acceptable plus if at the same time I don't really agree with proposed alternatives, I'll leave fixing the bug to someone else. Of course the expectation then is that such a bug fix come forward within a reasonable time frame ... Jan
