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

[Alex Williamson]

On Mon, 27 Apr 2020 08:58:18 -0300
Jason Gunthorpe <jgg@xxxxxxxxxxxx> wrote:

> 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.

Not sure what you're referring to, I see about 30 lines of code in
vdcm_vidxd_cfg_write() that specifically handle writes to the 4 BARs in
config space and maybe a couple hundred lines of code in total handling
config space emulation.  Thanks,

Alex