Messages in this thread | | | Subject | Re: [PATCH RFC v2 00/18] Add VFIO mediated device support and DEV-MSI support for the idxd driver | From | Jason Wang <> | Date | Wed, 12 Aug 2020 11:28:12 +0800 |
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On 2020/8/10 下午3:32, Tian, Kevin wrote: >> From: Jason Gunthorpe <jgg@nvidia.com> >> Sent: Friday, August 7, 2020 8:20 PM >> >> On Wed, Aug 05, 2020 at 07:22:58PM -0600, Alex Williamson wrote: >> >>> If you see this as an abuse of the framework, then let's identify those >>> specific issues and come up with a better approach. As we've discussed >>> before, things like basic PCI config space emulation are acceptable >>> overhead and low risk (imo) and some degree of register emulation is >>> well within the territory of an mdev driver. >> What troubles me is that idxd already has a direct userspace interface >> to its HW, and does userspace DMA. The purpose of this mdev is to >> provide a second direct userspace interface that is a little different >> and trivially plugs into the virtualization stack. > No. Userspace DMA and subdevice passthrough (what mdev provides) > are two distinct usages IMO (at least in idxd context). and this might > be the main divergence between us, thus let me put more words here. > If we could reach consensus in this matter, which direction to go > would be clearer. > > First, a passthrough interface requires some unique requirements > which are not commonly observed in an userspace DMA interface, e.g.: > > - Tracking DMA dirty pages for live migration; > - A set of interfaces for using SVA inside guest; > * PASID allocation/free (on some platforms); > * bind/unbind guest mm/page table (nested translation); > * invalidate IOMMU cache/iotlb for guest page table changes; > * report page request from device to guest; > * forward page response from guest to device; > - Configuring irqbypass for posted interrupt; > - ... > > Second, a passthrough interface requires delegating raw controllability > of subdevice to guest driver, while the same delegation might not be > required for implementing an userspace DMA interface (especially for > modern devices which support SVA). For example, idxd allows following > setting per wq (guest driver may configure them in any combination): > - put in dedicated or shared mode; > - enable/disable SVA; > - Associate guest-provided PASID to MSI/IMS entry; > - set threshold; > - allow/deny privileged access; > - allocate/free interrupt handle (enlightened for guest); > - collect error status; > - ... > > We plan to support idxd userspace DMA with SVA. The driver just needs > to prepare a wq with a predefined configuration (e.g. shared, SVA, > etc.), bind the process mm to IOMMU (non-nested) and then map > the portal to userspace. The goal that userspace can do DMA to > associated wq doesn't change the fact that the wq is still *owned* > and *controlled* by kernel driver. However as far as passthrough > is concerned, the wq is considered 'owned' by the guest driver thus > we need an interface which can support low-level *controllability* > from guest driver. It is sort of a mess in uAPI when mixing the > two together.
So for userspace drivers like DPDK, it can use both of the two uAPIs?
> > Based on above two reasons, we see distinct requirements between > userspace DMA and passthrough interfaces, at least in idxd context > (though other devices may have less distinction in-between). Therefore, > we didn't see the value/necessity of reinventing the wheel that mdev > already handles well to evolve an simple application-oriented usespace > DMA interface to a complex guest-driver-oriented passthrough interface. > The complexity of doing so would incur far more kernel-side changes > than the portion of emulation code that you've been concerned about... > >> I don't think VFIO should be the only entry point to >> virtualization. If we say the universe of devices doing user space DMA >> must also implement a VFIO mdev to plug into virtualization then it >> will be alot of mdevs. > Certainly VFIO will not be the only entry point. and This has to be a > case-by-case decision.
The problem is that if we tie all controls via VFIO uAPI, the other subsystem like vDPA is likely to duplicate them. I wonder if there is a way to decouple the vSVA out of VFIO uAPI?
> If an userspace DMA interface can be easily > adapted to be a passthrough one, it might be the choice.
It's not that easy even for VFIO which requires a lot of new uAPIs and infrastructures(e.g mdev) to be invented.
> But for idxd, > we see mdev a much better fit here, given the big difference between > what userspace DMA requires and what guest driver requires in this hw.
A weak point for mdev is that it can't serve kernel subsystem other than VFIO. In this case, you need some other infrastructures (like [1]) to do this.
(For idxd, you probably don't need this, but it's pretty common in the case of networking or storage device.)
Thanks
[1] https://patchwork.kernel.org/patch/11280547/
> >> I would prefer to see that the existing userspace interface have the >> extra needed bits for virtualization (eg by having appropriate >> internal kernel APIs to make this easy) and all the emulation to build >> the synthetic PCI device be done in userspace. > In the end what decides the direction is the amount of changes that > we have to put in kernel, not whether we call it 'emulation'. For idxd, > adding special passthrough requirements (guest SVA, dirty tracking, > etc.) and raw controllability to the simple userspace DMA interface > is for sure making kernel more complex than reusing the mdev > framework (plus some degree of emulation mockup behind). Not to > mention the merit of uAPI compatibility with mdev... > > Thanks > Kevin >
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