Re: [RFC 0/4] vfio-pci/zdev: Fixing s390 vfio-pci ISM support

[Cornelia Huck <cohuck@redhat.com>]

On Thu, 10 Dec 2020 10:51:23 -0500
Matthew Rosato <mjrosato@linux.ibm.com> wrote:

> On 12/10/20 7:33 AM, Cornelia Huck wrote:
> > On Wed,  9 Dec 2020 15:27:46 -0500
> > Matthew Rosato <mjrosato@linux.ibm.com> wrote:
> >   
> >> Today, ISM devices are completely disallowed for vfio-pci passthrough as
> >> QEMU will reject the device due to an (inappropriate) MSI-X check.
> >> However, in an effort to enable ISM device passthrough, I realized that the
> >> manner in which ISM performs block write operations is highly incompatible
> >> with the way that QEMU s390 PCI instruction interception and
> >> vfio_pci_bar_rw break up I/O operations into 8B and 4B operations -- ISM
> >> devices have particular requirements in regards to the alignment, size and
> >> order of writes performed.  Furthermore, they require that legacy/non-MIO
> >> s390 PCI instructions are used, which is also not guaranteed when the I/O
> >> is passed through the typical userspace channels.  
> > 
> > The part about the non-MIO instructions confuses me. How can MIO
> > instructions be generated with the current code, and why does changing  
> 
> So to be clear, they are not being generated at all in the guest as the 
> necessary facility is reported as unavailable.
> 
> Let's talk about Linux in LPAR / the host kernel:  When hardware that 
> supports MIO instructions is available, all userspace I/O traffic is 
> going to be routed through the MIO variants of the s390 PCI 
> instructions.  This is working well for other device types, but does not 
> work for ISM which does not support these variants.  However, the ISM 
> driver also does not invoke the userspace I/O routines for the kernel, 
> it invokes the s390 PCI layer directly, which in turn ensures the proper 
> PCI instructions are used -- This approach falls apart when the guest 
> ISM driver invokes those routines in the guest -- we (qemu) pass those 
> non-MIO instructions from the guest as memory operations through 
> vfio-pci, traversing through the vfio I/O layer in the guest 
> (vfio_pci_bar_rw and friends), where we then arrive in the host s390 PCI 
> layer -- where the MIO variant is used because the facility is available.
> 
> Per conversations with Niklas (on CC), it's not trivial to decide by the 
> time we reach the s390 PCI I/O layer to switch gears and use the non-MIO 
> instruction set.
> 
> > the write pattern help?  
> 
> The write pattern is a separate issue from non-MIO instruction 
> requirements...  Certain address spaces require specific instructions to 
> be used (so, no substituting PCISTG for PCISTB - that happens too by 
> default for any writes coming into the host s390 PCI layer that are 
> <=8B, and they all are when the PCISTB is broken up into 8B memory 
> operations that travel through vfio_pci_bar_rw, which further breaks 
> those up into 4B operations).  There's also a requirement for some 
> writes that the data, if broken up, be written in a certain order in 
> order to properly trigger events. :(  The ability to pass the entire 
> PCISTB payload vs breaking it into 8B chunks is also significantly faster.

Let me summarize this to make sure I understand this new region
correctly:

- some devices may have relaxed alignment/length requirements for
  pcistb (and friends?)
- some devices may actually require writes to be done in a large chunk
  instead of being broken up (is that a strict subset of the devices
  above?)
- some devices do not support the new MIO instructions (is that a
  subset of the relaxed alignment devices? I'm not familiar with the
  MIO instructions)

The patchsets introduce a new region that (a) is used by QEMU to submit
writes in one go, and (b) makes sure to call into the non-MIO
instructions directly; it's basically killing two birds with one stone
for ISM devices. Are these two requirements (large writes and non-MIO)
always going hand-in-hand, or is ISM just an odd device?

If there's an expectation that the new region will always use the
non-MIO instructions (in addition to the changed write handling), it
should be noted in the description for the region as well.