Re: Question about reserved_regions w/ Intel IOMMU

[Robin Murphy]

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:

https://lkml.org/lkml/2018/3/2/760

Thanks,
Robin.