On Sun, Apr 26, 2020 at 09:43:55PM -0600, Alex Williamson wrote: > On Sun, 26 Apr 2020 16:13:57 -0300 > Jason Gunthorpe <jgg@xxxxxxxxxxxx> wrote: > > > On Sun, Apr 26, 2020 at 05:18:59AM +0000, Tian, Kevin wrote: > > > > > > > I think providing an unified abstraction to userspace is also important, > > > > > which is what VFIO provides today. The merit of using one set of VFIO > > > > > API to manage all kinds of mediated devices and VF devices is a major > > > > > gain. Instead, inventing a new vDPA-like interface for every Scalable-IOV > > > > > or equivalent device is just overkill and doesn't scale. Also the actual > > > > > emulation code in idxd driver is actually small, if putting aside the PCI > > > > > config space part for which I already explained most logic could be shared > > > > > between mdev device drivers. > > > > > > > > If it was just config space you might have an argument, VFIO already > > > > does some config space mangling, but emulating BAR space is out of > > > > scope of VFIO, IMHO. > > > > > > out of scope of vfio-pci, but in scope of vfio-mdev. btw I feel that most > > > of your objections are actually related to the general idea of > > > vfio-mdev. > > > > There have been several abusive proposals of vfio-mdev, everything > > from a way to create device drivers to this kind of generic emulation > > framework. > > > > > Scalable IOV just uses PASID to harden DMA isolation in mediated > > > pass-through usage which vfio-mdev enables. Then are you just opposing > > > the whole vfio-mdev? If not, I'm curious about the criteria in your mind > > > about when using vfio-mdev is good... > > > > It is appropriate when non-PCI standard techniques are needed to do > > raw device assignment, just like VFIO. > > > > Basically if vfio-pci is already doing it then it seems reasonable > > that vfio-mdev should do the same. This mission creep where vfio-mdev > > gains functionality far beyond VFIO is the problem. > > Ehm, vfio-pci emulates BARs too. We also emulate FLR, power > management, DisINTx, and VPD. FLR, PM, and VPD all have device > specific quirks in the host kernel, and I've generally taken the stance > that would should take advantage of those quirks, not duplicate them in > userspace and not invent new access mechanisms/ioctls for each of them. > Emulating DisINTx is convenient since we must have a mechanism to mask > INTx, whether it's at the device or the APIC, so we can pretend the > hardware supports it. BAR emulation is really too trivial to argue > about, the BARs mean nothing to the physical device mapping, they're > simply scratch registers that we mask out the alignment bits on read. > vfio-pci is a mix of things that we decide are too complicated or > irrelevant to emulate in the kernel and things that take advantage of > shared quirks or are just too darn easy to worry about. BARs fall into > that latter category, any sort of mapping into VM address spaces is > necessarily done in userspace, but scratch registers that are masked on > read, *shrug*, vfio-pci does that. Thanks, It is not trivial masking. It is a 2000 line patch doing comprehensive emulation. Jason
