On Mon, Jun 19, 2023 at 11:20:58AM +0100, Robin Murphy wrote: > On 2023-06-16 19:59, Jason Gunthorpe wrote: > > On Fri, Jun 16, 2023 at 05:34:53PM +0100, Robin Murphy wrote: > > > > > > If the system has working ACS configured correctly, then this issue should > > > be moot; > > > > Yes > > > > > if it doesn't, then a VFIO user is going to get a whole group of > > > peer devices if they're getting anything at all, so it doesn't seem entirely > > > unreasonable to leave it up to them to check that all those devices' > > > resources play well with their expected memory map. > > > > I think the kernel should be helping here.. 'go figure it out from > > lspci' is a very convoluted and obscure uAPI, and I don't see things > > like DPDK actually doing that. > > > > IMHO the uAPI expectation is that the kernel informs userspace what > > the usable IOVA is, if bridge windows and lack of ACS are rendering > > address space unusable then VFIO/iommufd should return it as excluded > > as well. > > > > If we are going to do that then all UNAMANGED domain users should > > follow the same logic. > > > > We probably have avoided bug reports because of how rare it would be > > to see a switch and an UNMANAGED domain using scenario together - > > especially with ACS turned off. > > > > So it is really narrow niche.. Obscure enough I'm not going to make > > patches :) > > The main thing is that we've already been round this once before; we tried > it 6 years ago and then reverted it a year later for causing more problems > than it solved: As I said earlier in this thread if we do it for VFIO then the calculation must be precise and consider bus details like ACS/etc. eg VFIO on an ACS system should not report any new regions. It looks like that thread confirms we can't create reserved regions which are wrong :) I think Alex is saying the same things I'm saying in that thread too: https://lore.kernel.org/all/20180226161310.061ce3a8@xxxxxxxxx/ (b) is what the kernel should help prevent. And it is clear there are today scenarios where a VFIO user will get data loss because the reported valid IOVA from the kernel is incorrect. Fixing this is hard, much harder than what commit 273df9635385 ("iommu/dma: Make PCI window reservation generic") has. Thanks, Jason
