Re: Question about reserved_regions w/ Intel IOMMU

[Jason Gunthorpe]

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