Re: [PATCH RFC 00/15] Add VFIO mediated device support and IMS support for the idxd driver.

[Jason Gunthorpe <jgg@mellanox.com>]

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@mellanox.com> 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