Re: [dpdk-dev] [PATCH 2/2] eal: fix IOVA mode selection as VA for pci drivers

[Thomas Monjalon <thomas@monjalon.net>]

12/07/2019 13:03, Burakov, Anatoly:
> On 10-Jul-19 10:48 PM, David Marchand wrote:
> > The incriminated commit broke the use of RTE_PCI_DRV_IOVA_AS_VA which
> > was intended to mean "driver only supports VA" but had been understood
> > as "driver supports both PA and VA" by most net drivers and used to let
> > dpdk processes to run as non root (which do not have access to physical
> > addresses on recent kernels).
> > 
> > The check on physical addresses actually closed the gap for those
> > drivers. We don't need to mark them with RTE_PCI_DRV_IOVA_AS_VA and this
> > flag can retain its intended meaning.
> > Document explicitly its meaning.
> > 
> 
> So, we always assume that all devices support both IOVA as PA and IOVA 
> as VA by default. Well, as long as it's understood and documented :)

Yes
Please make sure it is well documented.

> Unless...
> 
> 
> <snip>
> 
> > +
> > +IOVA Mode is selected by considering what the current usable Devices on the
> > +system requires and/or supports.
> > +
> > +Below is the 2-step heuristic for this choice.
> > +
> > +For the first step, EAL asks each bus its requirement in terms of IOVA mode
> > +and decides on a preferred IOVA mode.
> > +
> > +- if all buses report RTE_IOVA_PA, then the preferred IOVA mode is RTE_IOVA_PA,
> > +- if all buses report RTE_IOVA_VA, then the preferred IOVA mode is RTE_IOVA_VA,
> > +- if all buses report RTE_IOVA_DC, no bus expressed a preferrence, then the
> > +  preferred mode is RTE_IOVA_DC,
> > +- if the buses disagree (at least one wants RTE_IOVA_PA and at least one wants
> > +  RTE_IOVA_VA), then the preferred IOVA mode is RTE_IOVA_DC (see below with the
> > +  check on Physical Addresses availability),
> > +
> > +The second step is checking if the preferred mode complies with the Physical
> > +Addresses availability since those are only available to root user in recent
> > +kernels.
> > +
> > +- if the preferred mode is RTE_IOVA_PA but there is no access to Physical
> > +  Addresses, then EAL init will fail early, since later probing of the devices
> > +  would fail anyway,
> > +- if the preferred mode is RTE_IOVA_DC then based on the Physical Addresses
> > +  availability, the preferred mode is adjusted to RTE_IOVA_PA or RTE_IOVA_VA.
> > +  In the case when the buses had disagreed on the IOVA Mode at the first step,
> > +  part of the buses won't work because of this decision.
> 
> Is there any specific reason why we always prefer PA if physical 
> addresses are available? Since we're already assuming that all devices 
> support PA and VA anyway, what's the harm in enabling VA by default?

If PA is available, it means we are running as root.
We can assume that using root is a choice, probably related
to a preference for PA.

> I seem to recall there were some concerns around SPDK and PA address 
> availability - doesn't that mean that the assumption regarding PA and VA 
> mode always being supported doesn't actually hold in practice?
> 
> By the way, the reason i'm harping away on IOVA as VA being the default 
> is because having IOVA as PA is not a free (as in beer) choice - we 
> sacrifice some usability by doing that. Right now, by default, mempool 
> will ask for IOVA-contiguous memory first, and this is slow in IOVA as 
> PA mode - meaning, e.g. testpmd startup time is greatly increased for 
> smaller page sizes because of IOVA as PA mode is the default in DPDK.
> 
> I would also like to steer people away from using real physical 
> addresses because doing so while requiring lots of IOVA contiguous 
> memory also requires legacy mem mode, which i would rather people not 
> use and grow dependent on, and would like to remove it at some point as 
> it adds a lot of complexity for a corner case.

That's why we should better encourage to not run as root.
We need more documentation about how to run as normal user.

> So, picking address mode is not *just* about whether the device supports 
> them - it has usability implications as well.

If we consider running as root an exception, then it makes
sense to pick address mode which fits this exception (PA).