Re: [dpdk-dev] [EXT] Re: [PATCH] bus/pci: fix IOVA as VA mode selection

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

> -----Original Message-----
> From: Burakov, Anatoly <anatoly.burakov@intel.com>
> Sent: Tuesday, July 9, 2019 5:10 PM
> To: Jerin Jacob Kollanukkaran <jerinj@marvell.com>; David Marchand
> <david.marchand@redhat.com>
> Cc: dev <dev@dpdk.org>; Thomas Monjalon <thomas@monjalon.net>; Ben
> Walker <benjamin.walker@intel.com>
> Subject: Re: [EXT] Re: [dpdk-dev] [PATCH] bus/pci: fix IOVA as VA mode
> selection
> >>> ________________________________________
> >>>
> >>> On Mon, Jul 8, 2019 at 4:25 PM <mailto:jerinj@marvell.com> wrote:
> >>> From: Jerin Jacob <mailto:jerinj@marvell.com>
> >>>
> >>> Existing logic fails to select IOVA mode as VA if driver request to
> >>> enable IOVA as VA.
> >>>
> >>> IOVA as VA has more strict requirement than other modes, so enabling
> >>> positive logic for IOVA as VA selection.
> >>>
> >>> This patch also updates the default IOVA mode as PA for PCI devices
> >>> as it has to deal with DMA engines unlike the virtual devices that
> >>> may need only IOVA as DC.
> >>>
> >>> We have three cases:
> >>> - driver/hw supports IOVA as PA only
> >>>
> >>> [Jerin] It is not driver cap, it is more of system cap(IOMMU vs non
> >>> IOMMU). We are already addressing that case
> >>
> >> I don't get how this works. How does "system capability" affect what
> >> the device itself supports? Are we to assume that *all* hardware
> >> support IOVA as VA by default? "System capability" is more of a bus
> >> issue than an individual device issue, is it not?
> >
> > What I meant is, supporting VA vs PA is function of IOMMU(not the device
> attribute).
> > Ie. Device makes the  bus master request, if IOMMU available and
> > enabled in the SYSTEM , It goes over IOMMU  and translate the IOVA to
> physical address.
> >
> > Another way to put is, Is there any _PCIe_ device which need/requires
> > RTE_PCI_DRV_NEED_IOVA_AS_PA in rte_pci_driver.drv_flags
> >
> >
> 
> Previously, as far as i can tell, the flag was used to indicate support for IOVA
> as VA mode, not *requirement* for IOVA as VA mode. For example, there
> are multiple patches [1][2][3][4] (i'm sure i can find more!) that added IOVA
> as VA support to various drivers, and they all were worded it in this exact way
> - "support for IOVA as VA mode", not "require IOVA as VA mode". As far as i
> can tell, none of these drivers *require* IOVA as VA mode - they merely use
> this flag to indicate support for it.

Some class of devices NEED IOVA as VA for performance reasons.
Specially the devices has HW mempool allocators. On those devices If we don’t use IOVA as VA,
Upon getting packet from device, It needs to go over rte_mem_iova2virt() per 
packet see driver/net/dppa2. Which has real performance issue.

> Specifically, from my perspective, the "support for IOVA as VA mode" has in
> practice always indicated support for VFIO (or similar drivers) as far as the PCI
> bus is concerned. As in, the device *could* use IOVA as VA mode, but since it
> may be bound to igb_uio (which doesn't support IOVA as VA), the IOVA as
> VA mode may not be supported for a particular device. So, a particular device
> *cannot support* IOVA as VA if it's bound to igb_uio or uio_pci_generic (or
> VFIO in noiommu mode). This is not *just* a capability thing, but also kernel
> driver issue.

Yes. See below.

> 
> Now suddenly it turns out that someone somewhere "knew" that "IOVA as
> VA" flag in PCI drivers is supposed to indicate *requirement* and not
> support, and it appears that this knowledge was not communicated nor
> documented anywhere, and is now treated as common knowledge.

I think, the confusion here is,  I was under impression that
# If device supports IOVA as VA and system runs with IOMMU then
the  dpdk should run in IOVA as VA mode.
If above statement true then we don’t really need a new flag.

Couple of points to make forward progress:
# If we think, there is a use case where device is IOVA as VA 
And system runs in IOMMU mode then for some reason DPDK needs
to run in PA mode. If so, we need to create two flags
RTE_PCI_DRV_IOVA_AS_VA - it can run either modes
RTE_PCI_DRV_NEED_IOVA_AS_VA - it can run only on IOVA as VA
# With top of tree, Currently it never runs in IOVA as VA mode.
That’s a separate problem to fix. Which effect all the devices
Currently supporting RTE_PCI_DRV_IOVA_AS_VA. Ie even though
Device support RTE_PCI_DRV_IOVA_AS_VA, it is not running
With IOMMU protection and/or root privilege is required to run DPDK.


> 
> [1] http://patchwork.dpdk.org/patch/53206/
> [2] http://patchwork.dpdk.org/patch/50274/
> [3] http://patchwork.dpdk.org/patch/50991/
> [4] http://patchwork.dpdk.org/patch/46134/
> 
> --
> Thanks,
> Anatoly