RE: [PATCH v3 07/17] iommufd: Add IOMMU_RESV_IOVA_RANGES

[Tian, Kevin]

> From: Jason Gunthorpe <jgg@xxxxxxxxxx>
> Sent: Monday, July 31, 2023 9:24 PM
> 
> On Mon, Jul 31, 2023 at 06:21:44AM +0000, Tian, Kevin wrote:
> 
> > > As it is, userspace will have to aggregate the list, sort it, merge
> > > adjacent overlapping reserved ranges then invert the list to get an
> > > allowed list. This is not entirely simple..
> > >
> > > Did you already write an algorithm to do this in qemu someplace?
> >
> > Qemu is optional to aggregate it for S2 given IOMMU_IOAS_IOVA_RANGES
> > is still being used. If the only purpose of using this new cmd is to report
> > per-device reserved ranges to the guest then aggregation is not required.
> 
> I don't think it is entirely optional.. If qmeu doesn't track this,
> then it will have failures when attaching the S2 to the device. It
> needs to make sure it punches the right holes in the guest memory map
> to be compatible with the VFIO HW.
> 
> I suppose in reality the reserved regions are fairly predictable and
> probably always match the existing qemu memory map so you can ignore
> this and still work.
> 
> Plus most qemu cases don't deal with hotplug so you can build up the
> identity ioas with all the devices and then use IOMMU_IOAS_IOVA_RANGES
> as you say and still work.
> 
> > Arguably IOMMU_IOAS_IOVA_RANGES becomes redundant with this
> > new cmd. But it's already there and as you said it's actually more
> > convenient to be used if the user doesn't care about per-device
> > reserved ranges...
> 
> Yes and yes
> 
> So, I guess we should leave it like this?
> 

Yes. Along with this discussion (including what you explained for sw_msi)
let's abandon this new cmd and leave it as today.