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

[Jerin Jacob Kollanukkaran <jerinj@marvell.com>]

> > > +
> > > +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.

# Even if we are running as root, Why to choose PA in case of DC?
ie. Following logic is not need
                if (iova_mode == RTE_IOVA_DC) {
                        iova_mode = phys_addrs ? RTE_IOVA_PA : RTE_IOVA_VA;
                        RTE_LOG(DEBUG, EAL,
                                "Buses did not request a specific IOVA mode, using '%s' based on physical addresses availability.\n",
                                phys_addrs ? "PA" : "VA");
                }
# When DPDK running on guest, Anyway it can not access the real PA, It will be IPA.
So I don't understand logic behind choose PA when DC. To me, it make sense to choose PA when DC.
# To align with RTE_PCI_DRV_NEED_MAPPING flag and reflect it "need" rather
than support, I think, flag can be changed to RTE_PCI_DRV_NEED_IOVA_AS_VA

Other than above points,
Reviewed this patch and tested on octeontx2, It looks good to me.