Re: [dpdk-dev] [PATCH] raw/ioat: fix bus requiring virtual addressing when no devs

[Thomas Monjalon <thomas@monjalon.net>]

06/05/2021 17:34, Bruce Richardson:
> On Thu, May 06, 2021 at 04:27:03PM +0100, Burakov, Anatoly wrote:
> > On 06-May-21 4:09 PM, Bruce Richardson wrote:
> > > If after a bus scan, there are no devices using a particular bus, then
> > > that bus should not be taken into account when deciding whether DPDK
> > > should be run in VA or PA addressing mode. This becomes an issue when
> > > the DSA bus driver code is used on a system without an IOMMU. The PCI
> > > bus correctly reports that it only works in PA mode, while the DSA bus -
> > > also correctly - reports that it works only in VA mode. The difference
> > > is that there will be no devices found in a scan for the DSA bus, since
> > > the kernel driver can only present those to userspace in the presence of
> > > an IOMMU.
> > > 
> > > While we could change DSA instance to always report that it does not
> > > care about the addressing mode, this would imply that it could be used
> > > with DPDK in PA mode which is not the case. Therefore, this patch
> > > changes the driver to report DC (don't care) in the case where no
> > > devices are present, and VA otherwise.
> > > 
> > > NOTE: this addressing mode use of VA-only applies only in the case of
> > > using DSA through the idxd kernel driver. The use of DSA though vfio-pci
> > > is unaffected and works as with other PCI devices.
> > > 
> > > Fixes: b7aaf417f936 ("raw/ioat: add bus driver for device scanning automatically")
> > > 
> > > Reported-by: Harry van Haaren <harry.van.haaren@intel.com>
> > > Signed-off-by: Bruce Richardson <bruce.richardson@intel.com>
> > > ---
> > 
> > This would be a good opportunity to start a discussion on a "proper" fix for
> > this, because while this hack is acceptable being so close to release, IMO
> > we need a more general solution. Maybe a bus API reporting number of scanned
> > devices would be nice to have, as EAL will then be able to ignore the
> > opinion of those buses who don't have any devices.
> > 
> > The "number of devices" is a bit of a fluid concept, as number of scanned
> > devices may not be the same as number of *probed* devices, and there's also
> > hotplug. However, i think that we can ignore all that, because we care about
> > initialization only - any other issues can be addressed by forcing
> > --iova-mode by the user, as we cannot really do anything about hotplug or
> > devices that fail to probe.
> > 
> > Thoughts?
> > 
> Perhaps rather than just "number of devices" we should have buses report
> whether they are relevant or not. For example, there are a number of
> SoC-specific bus drivers in DPDK, which are presumably only relevant on
> particular SoCs. Similarly here for the DSA bus, the presence of the
> devices is a useful proxy for relevance - especially since it doesn't
> support hotplug - but if we had a way to report relevance/non-relevance DSA
> could just report itself as not-relevant in case where there is no IOMMU
> enabled on the system, or when the "idxd" kernel driver is not present, for
> example, and then have EAL skip even asking about PA/VA support

I think what you are describing is the same as reporting "don't care".
Why do you consider it as a hack?