Hi Jason, Joao, >-----Original Message----- >From: Jason Gunthorpe <j...@nvidia.com> >Sent: Thursday, November 9, 2023 10:35 PM >Subject: Re: [PATCH v4 28/41] vfio/iommufd: Implement the iommufd backend > >On Thu, Nov 09, 2023 at 01:21:59PM +0000, Joao Martins wrote: >> On 09/11/2023 13:09, Jason Gunthorpe wrote: >> > On Thu, Nov 09, 2023 at 01:03:02PM +0000, Joao Martins wrote: >> > >> >>> I am not talking about mdevs; but rather the regular (non mdev) case not >being >> >>> able to use dirty tracking with autodomains hwpt allocation. >> >> >> >> ... without any vIOMMU. >> > >> > Ah, well, that is troublesome isn't it.. >> > >> > So do we teach autodomains to be more featured in the kernel or do we >> > teach the generic qemu code to effectively implement autodomains in >> > userspace? >> >> The latter is actually what we have been doing. Well I wouldn't call >autodomains >> in qemu, but rather just allocate a hwpt, instead of attaching the IOAS >> directly. But well mdevs don't have domains and we overlooked that. I would >turn >> the exception into an exception rather than making the norm, doesn't look to >be >> much complexity added? > >Autodomains are complex because of things like mdev and iommu >non-uniformity's. Qemu can't just allocate a single HWPT, it needs to >be annoyingly managed. > >> What I last re-collect is that autodomains represents the 'simple users' that >> don't care much beyond the basics of IOMMU features (I recall the example >was >> DPDK apps and the like). You could say that for current needs IOMMU >autodomains >> suffices for qemu. > >Yes, that was my intention. Aside from that it primarily exists to >support vfio compatibility > >> Connecting autodomains to this enforcing on the hwpt is relatively simple >> btw, >> it just needs to connect the dirty tracking flag with same semantic of >> hwpt-alloc equivalent and pass the hwpt flags into the domain allocation. > >Yes > >> It's more of what of a question should be the expectations to the user when >> using ATTACH_HWPT with an IOAS_ID versus direct manipulation of HWPT. I am >> wondering if dirty tracking is alone here or whether there's more features >> that >> start to mud the simplicity of autodomains that would approximate of hwpt- >alloc. > >This is why I had been thinking of a pure HWPT based scheme > >So it seems we cannot have a simple model where the generic qmeu layer >just works in IOAS :( It might as well always work in HWPT and >understand all the auto domains complexity itself.
Let me know if there is anything I can do in this series to facilitate future qemu dirty tracking support of iommufd. Not clear if I should restore to the manual HWPT_ALLOC method in v4. Thanks Zhenzhong
