On 09-Jul-19 1:11 PM, Jerin Jacob Kollanukkaran wrote:
-----Original Message-----
From: Burakov, Anatoly <anatoly.bura...@intel.com>
Sent: Tuesday, July 9, 2019 5:10 PM
To: Jerin Jacob Kollanukkaran <jer...@marvell.com>; David Marchand
<david.march...@redhat.com>
Cc: dev <dev@dpdk.org>; Thomas Monjalon <tho...@monjalon.net>; Ben
Walker <benjamin.wal...@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:jer...@marvell.com> wrote:
From: Jerin Jacob <mailto:jer...@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.
I wouldn't classify this as "needing" IOVA. "Need" implies it cannot
work without it, whereas in this case it's more of a "highly
recommended" rather than "need".
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.
Exactly. And the flag used to indicate that the device *supports* IOVA
as VA, not that it *requires* it.
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
There are use cases - KNI and igb_uio come to mind. Whether IOMMU uses
VA or PA is a different from whether IOMMU is in use - there is no law
that states that, when using IOMMU, IOVA have to have 1:1 mapping with
VA. IOMMU requirement does not necessarily imply IOVA as VA - it is
perfectly legal to program IOMMU to use IOVA as PA (which we currently
do when we e.g. use VFIO for some devices and igb_uio for others).
RTE_PCI_DRV_NEED_IOVA_AS_VA - it can run only on IOVA as VA
If we're adding a flag, we might as well not create a confusion and do
it consistently. If IOVA as PA is supported, have a flag to indicate
that. If IOVA as VA is supported, have a flag to indicate that. Absence
of either flag implies inability to work in that mode. I don't see how
this is less clear and self-documenting than having two IOVA as
VA-related flags that have slightly different meaning and imply things
not otherwise stated explicitly.
# 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
--
Thanks,
Anatoly
