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

From: Matthew Rosato
Date: Wed Jan 27 2021 - 13:28:31 EST

On 1/27/21 12:45 PM, Cornelia Huck wrote:
On Wed, 27 Jan 2021 08:53:05 -0700
Alex Williamson <alex.williamson@xxxxxxxxxx> wrote:

On Wed, 27 Jan 2021 09:23:04 -0500
Matthew Rosato <mjrosato@xxxxxxxxxxxxx> wrote:

On 1/26/21 6:18 PM, Alex Williamson 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..

Why is this device specific behavior? It would be a fair argument that
acceptable device access widths for MMIO are always device specific, so
we should never break them down. Looking at the PCI spec, a TLP
requires a dword (4-byte) aligned address with a 10-bit length field > indicating the number of dwords, so up to 4K data as you suggest is the

Well, as I mentioned in a different thread, it's not really device

Sorry, I tried to follow the thread, not sure it's possible w/o lots of
preexisting s390 knowledge.

specific behavior but rather architecture/s390x specific behavior;
PCISTB is an interface available to all PCI devices on s390x, it just so
happens that ISM is the first device type that is running into the
nuance. The architecture is designed in such a way that other devices
can use the same interface in the same manner.

As a platform access mechanism, this leans towards a platform specific
implementation of the PCI BAR regions.
To drill down a bit, the PCISTB payload can either be 'strict' or
'relaxed', which via the s390x architecture 'relaxed' means it's not
dword-aligned but rather byte-aligned up to 4K. We find out at init
time which interface a device supports -- So, for a device that
supports 'relaxed' PCISTB like ISM, an example would be a PCISTB of 11
bytes coming from a non-dword-aligned address is permissible, which
doesn't match the TLP from the spec as you described... I believe this
'relaxed' operation that steps outside of the spec is unique to s390x.
(Conversely, devices that use 'strict' PCISTB conform to the typical TLP
definition)

This deviation from spec is to my mind is another argument to treat
these particular PCISTB separately.

I think that's just an accessor abstraction, we're not asking users to
generate TLPs. If we expose a byte granularity interface, some
platforms might pass that directly to the PCISTB command, otherwise
might align the address, perform a dword access, and return the
requested byte. AFAICT, both conventional and express PCI use dword
alignement on the bus, so this should be valid and at best questions
whether ISM is really PCI or not.

The vibes I'm getting from ISM is that it is mostly a construct using
(one set of) the s390 pci instructions, which ends up being something
not entirely unlike a pci device... the question is how much things

Yep, that's a fair assessment.

like the 'relaxed' payload may also be supported by 'real' pci devices
plugged into an s390.

The architecture allows for it, but in practicality PCI devices being used on s390x whose drivers are meant to run on additional platforms besides s390x (ex: mlx) aren't going to use something like that unless 1) an s390x-specific exploitation is tacked on to the driver for some reason and 2) I think support for such behavior would need to be implemented in the kernel -- As really, our s390 kernel PCI logic today makes no special accommodations for relaxed-alignment I/O coming from drivers -- this is one of the reasons the host ISM driver makes s390 PCI calls directly rather than going through the standard kernel PCI interface (and subsequently why using an alternate delivery vehicle for pass through ISM that connects directly with the s390 PCI interface made sense to me)

'Relaxed' is very s390-specific and as far as I understand is only expected from a device type that is unique to s390 (like ISM) because of the way it deviates from standard PCI. And today, the only device that behaves in that way (relaxed) and lacks support for certain PCI operations / requires a special access method (e.g. no PCISTG allowed) is ISM - else we'd see more devices that require direct invocation of s390 PCI interfaces in the kernel.




whole payload. It's quite possible that the reason we don't have more
access width problems is that MMIO is typically mmap'd on other
platforms. We get away with using the x-no-mmap=on flag for debugging,
but it's not unheard of that the device also doesn't work quite
correctly with that flag, which could be due to access width or timing
difference.

So really, I don't see why we wouldn't want to maintain the guest
access width through QEMU and the kernel interface for all devices. It
seems like that should be our default vfio-pci implementation. I think
we chose the current width based on the QEMU implementation that was
already splitting accesses, and it (mostly) worked. Thanks,

But unless you think that allowing more flexibility than the PCI spec
dictates (byte-aligned up to 4K rather than dword-aligned up to 4K, see
above) this still wouldn't allow me to solve the issue I'm trying to
with this patch set.

As above, it still seems like an improvement to honor user access width
to the ability of the platform or bus/device interface. If ISM is
really that different from PCI in this respect, it only strengthens the
argument to make a separate bus driver derived from vfio-pci(-core) imo.

If you DO think allowing byte-aligned access up to 4K is OK, then I'm
still left with a further issue (@Niklas): I'm also using this
region-based approach to ensure that the host uses a matching interface
when talking to the host device (basically, s390x has two different
versions of access to PCI devices, and for ISM at least we need to
ensure that the same operation intercepted from the guest is being used
on the host vs attempting to 'upgrade', which always happens via the
standard s390s kernel PCI interfaces).

In the proposed vfio-pci-core library model, devices would be attached
to the most appropriate vfio bus driver, an ISM device might be bound
to a vfio-zpci-ism (heh, "-ism") driver on the host, standard device

That would be a nice name, at least :)

might simply be attached to vfio-pci.

I'm wondering what a good split would be there. ISTM that the devices
the vfio-pci-core split is written in mind with are still normal pci
devices, just with some extra needs that can be fulfilled via an aux
driver. ISM seems to need special treatment for normal accesses, but
does not hook into a separate framework. (If other zpci devices need
special accesses, would that then be a zpci framework?)


As of today, I don't believe we have other devices that -require- special access, as I mentioned above (no future guarantee I suppose). However the notion of an improved PCISTB payload size beyond ISM devices (regardless of 'strict' or 'relaxed') is desirable, but not strictly necessary, as it would be applicable to any PCI device whose driver wants to do writes >8B on s390x and would allow that operation to be done in "one shot."