Re: [PATCH 4/4] vfio-pci/zdev: Introduce the zPCI I/O vfio region

[Matthew Rosato]

On 1/25/21 10:42 AM, Cornelia Huck wrote:
> On Mon, 25 Jan 2021 09:40:38 -0500
> Matthew Rosato <mjrosato@xxxxxxxxxxxxx> wrote:
>
> > On 1/22/21 6:48 PM, Alex Williamson wrote:
> > > On Tue, 19 Jan 2021 15:02:30 -0500
> > > Matthew Rosato <mjrosato@xxxxxxxxxxxxx> wrote:
> > >    
> > > > Some s390 PCI devices (e.g. ISM) perform I/O operations that have very
> > > > specific requirements in terms of alignment as well as the patterns in
> > > > which the data is read/written. Allowing these to proceed through the
> > > > typical vfio_pci_bar_rw path will cause them to be broken in up in such a
> > > > way that these requirements can't be guaranteed. In addition, ISM devices
> > > > do not support the MIO codepaths that might be triggered on vfio I/O coming
> > > > from userspace; we must be able to ensure that these devices use the
> > > > non-MIO instructions.  To facilitate this, provide a new vfio region by
> > > > which non-MIO instructions can be passed directly to the host kernel s390
> > > > PCI layer, to be reliably issued as non-MIO instructions.
> > > >
> > > > This patch introduces the new vfio VFIO_REGION_SUBTYPE_IBM_ZPCI_IO region
> > > > and implements the ability to pass PCISTB and PCILG instructions over it,
> > > > as these are what is required for ISM devices.
> > >
> > > There have been various discussions about splitting vfio-pci to allow
> > > more device specific drivers rather adding duct tape and bailing wire
> > > for various device specific features to extend vfio-pci.  The latest
> > > iteration is here[1].  Is it possible that such a solution could simply
> > > provide the standard BAR region indexes, but with an implementation that
> > > works on s390, rather than creating new device specific regions to
> > > perform the same task?  Thanks,
> > >
> > > Alex
> > >
> > > [1]https://lore.kernel.org/lkml/20210117181534.65724-1-mgurtovoy@xxxxxxxxxx/
> > >    
> >
> > Thanks for the pointer, I'll have to keep an eye on this.  An approach
> > like this could solve some issues, but I think a main issue that still
> > remains with relying on the standard BAR region indexes (whether using
> > the current vfio-pci driver or a device-specific driver) is that QEMU
> > writes to said BAR memory region are happening in, at most, 8B chunks
> > (which then, in the current general-purpose vfio-pci code get further
> > split up into 4B iowrite operations).  The alternate approach I'm
> > proposing here is allowing for the whole payload (4K) in a single
> > operation, which is significantly faster.  So, I suspect even with a
> > device specific driver we'd want this sort of a region anyhow..
>
> I'm also wondering about device specific vs architecture/platform
> specific handling.
>
> If we're trying to support ISM devices, that's device specific
> handling; but if we're trying to add more generic things like the large
> payload support, that's not necessarily tied to a device, is it? For
> example, could a device support large payload if plugged into a z, but
> not if plugged into another machine? >

Yes, that's correct -- While ISM is providing the impetus and has a hard 
requirement for some of this due to the MIO instruction quirk, the 
mechanism being implemented here is definitely not ISM-specific -- it's 
more like an s390-wide quirk that could really benefit any device that 
wants to do large payloads (PCISTB).

And I think that ultimately goes back to why Pierre wanted to have QEMU 
be as permissive as possible in using the region vs limiting it only to 
ISM.