Thursday, November 15, 2018 1:00 PM, Burakov, Anatoly:
Subject: Re: [RFC] ethdev: introduce DMA memory mapping for external
memory
On 15-Nov-18 9:46 AM, Shahaf Shuler wrote:
Wednesday, November 14, 2018 7:06 PM, Burakov, Anatoly:
Subject: Re: [RFC] ethdev: introduce DMA memory mapping for external
memory
On 14-Nov-18 2:53 PM, Shahaf Shuler wrote:
Hi Anatoly,
Wednesday, November 14, 2018 1:19 PM, Burakov, Anatoly:
Subject: Re: [RFC] ethdev: introduce DMA memory mapping for
external memory
Hi Shahaf,
Great to see such effort! Few comments below.
Note: halfway through writing my comments i realized that i am
starting with an assumption that this API is a replacement for
current VFIO DMA mapping API's. So, if my comments seem out of left
field, this is probably why :)
On 04-Nov-18 12:41 PM, Shahaf Shuler wrote:
Request for comment on the high level changes present on this patch.
The need to use external memory (memory belong to application and
not part of the DPDK hugepages) is allready present.
Starting from storage apps which prefer to manage their own memory
blocks for efficient use of the storage device. Continue with GPU
based application which strives to achieve zero copy while
processing the packet payload on the GPU core. And finally by
vSwitch/vRouter application who just prefer to have a full control
over the memory in use
(e.g. VPP).
Recent work[1] in the DPDK enabled the use of external memory,
however it mostly focus on VFIO as the only way to map memory.
While VFIO is common, there are other vendors which use different
ways to map memory (e.g. Mellanox and NXP[2]).
The work in this patch moves the DMA mapping to vendor agnostic
APIs located under ethdev. The choice in ethdev was because
memory
map should be associated with a specific port(s). Otherwise the
memory is being mapped multiple times to different frameworks and
ends up with memory being wasted on redundant translation table in
the host or in the
device.
So, anything other than ethdev (e.g. cryptodev) will not be able to
map memory for DMA?
That's is a fair point.
I have thought about this for some length of time, and i think DMA
mapping belongs in EAL (more specifically, somewhere at the bus
layer), rather than at device level.
I am not sure I agree here. For example take Intel and Mellanox devices.
Both are PCI devices, so how will you distinguish which mapping API to
use?
Also I still think the mapping should be in device granularity and
not
bus/system granularity, since it is very typical for a memory to be
used for DMA be a specific device.
Maybe we can say the DMA mapping is a rte_device attribute. It is
the
parent class for all the DPDK devices.
We need to see w/ vport representors (which all has the same
rte_device).
On that case I believe the rte_device.map call can register the
memory to all of the representors as well (if needed).
Placing this functionality at device level comes with more work
to support different device types and puts a burden on device
driver developers to implement their own mapping functions.
The mapping function can be shared. For example we can still
maintain the
vfio mapping scheme as part of eal and have all the related driver to
call this function.
The only overhead will be to maintain the function pointer for the dma
call.
With this work, instead of the eal layer to guess which type of DMA
mapping the devices in the system needs or alternatively force them
all to work w/ VFIO, each driver will select its own function.
The driver is the only one which knows what type of DMA mapping its
device needs.
However, i have no familiarity with how MLX/NXP devices do their
DMA mapping, so maybe the device-centric approach would be better.
We could provide "standard" mapping functions at the bus level
(such as VFIO mapping functions for PCI bus), so that this could
would not have to be reimplemented in the devices.
Yes, like I said above, I wasn't intending to re-implement all the
mapping
function again on each driver. Yet, I believe it should be per device.
Moreover, i'm not sure how this is going to work for VFIO. If this
is to be called for each NIC that needs access to the memory, then
we'll end up with double mappings for any NIC that uses VFIO,
unless you want each NIC to be in a separate container.
I am not much familiar w/ VFIO (you are the expert😊).
What will happen if we map the same memory twice (under same
container)? The translation on the IOMMU will be doubled? The map
will return with error that this memory mapping already exists?
The latter. You can't map the same memory twice in the same container.
You can't even keep NICs in separate containers because then
secondary processes won't work without serious rework.
So, all VFIO-mapped things will need to share the mappings.
It's not an insurmountable problem, but if we're going to share
mapping status for VFIO (i.e. track which area is already mapped),
then what's stopping us from doing the same for other DMA mapping
mechanisms? I.e.
instead of duplicating the mappings in each driver, provide some kind
of mechanism for devices to share the DMA mapping caches. Apologies
if i'm talking nonsense - i'm completely unfamiliar with how DMA
mapping works for MLX/NXP devices :)
Unfortunately it cannot be done at least w/ Mellanox.
In Mellanox the kernel driver is the one which maps the memory. The
mapping returns a key which identify a memory region which was just
registered to the device.
There is a complete separation between the ports, meaning one port
mapping cannot be used by in the other port, even if the key is known.
The separation is not only in ports, but also in processes (two primary ones,
for secondary we have a way to share). If two process work on the same
device, the must register the memory independently.
Ah, OK.
So, we're right back to where we started. Right now, external memory
expects to behave the same way as all other memory - you don't need to
perform DMA mapping for it.
That said, part of the reason *why* it was done that way was because there
is no way to trigger VFIO DMA mapping for NXP (or was it MLX?) devices. If
you look at initial versions of the patchset, the DMA mapping was actually
done manually. Then, i became convinced that doing this automatically is the
way to go, both because it erases the usability differences as far as memory
types are concerned, and because it enables whatever services that are
subscribing to memory events to receive notifications about external
memory as well (i.e. consistency).
Given that it's still an experimental API, we can tinker with it all we like,
so it's
not set in stone. However, i would really like to keep the current automagic
thing, because DMA mapping may not be the only user of memory callbacks -
they can be used for debug purposes, or for any other things.
Memory callbacks are good to have regardless.
The question we need to answer is whether or not we are going to provide the
DMA map abstraction for external memory. See more below.
For example, consider a host with Mellanox and Intel devices.
Mapping a memory without specifying to which port will end up with
IOMMU registration and Verbs (Mellanox DMA map) registration.
Another example can be two Mellanox devices on the same host. The
memory
will be mapped for both, even though application will use mempool
per device.
To use the suggested APIs the application will allocate a memory
block and will call rte_eth_dma_map. It will map it to every port
that needs DMA access to this memory.
This bit is unclear to me. What do you mean "map it to every port
that needs DMA access to this memory"? I don't see how this API
solves the above problem of mapping the same memory to all devices.
How does a device know which memory it will need? Does the user
specifically have to call this API for each and every NIC they're using?
Yes, the user will call this API for every port which needs to have
DMA
access to this memory.
Remember we are speaking here on external memory the application
allocated and wants to use for send/receive. The device doesn't
guess which memory he will need, the user is telling it to him explicitly.
For DPDK-managed memory, everything will still get mapped to every
device automatically, correct?
Yes, even though it is not the case today.
What do you mean it is not the case? It is the case today. When
external memory chunk is registered at the heap, a mem event callback
is triggered just like for regular memory, and this chunk does get
mapped to VFIO as well as any other subscribed entity. As i recall,
NXP NICs currently are set up to ignore externally allocated memory,
but for the general VFIO case, everything is mapped automatically.
If so, then such a manual approach for
external memory will be bad for both usability and drop-in
replacement of internal-to-external memory, because it introduces
inconsistency between using internal and external memory. From my
point of view, either we do *everything* manually (i.e. register
all memory for DMA
explicitly) and thereby avoid this problem but keep the
consistency, or we do *everything* automatically and deal with
duplication of mappings somehow (say, by MLX/NXP drivers sharing
their mappings through bus interface).
I understand your point, however I am not sure external and internal
memory *must* be consist.
The DPDK-managed memory is part of the DPDK subsystems and the
DPDK
libs are preparing it for the optimal use of the underlying devices.
The external memory is different, it is a proprietary memory the
application allocated and the DPDK cannot do anything in advance on it.
My view for designing external memory support was that it should
behave like regular DPDK memory for all intents and purposes, and be
a drop-in replacement, should you choose to use it. I.e. the
application should not care whether it uses internal or external
memory - it all sits in the same malloc heaps, it all uses the same
socket ID mechanisms, etc. - for all intents and purposes, they're one and
the same.
Even today there is inconsistency, because if user wants to use
external
memory it must map it (rte_vfio_dma_map) while he doesn't need to do
that for the DPDK-managed memory.
Well, now i see where your confusion stems from :) You didn't know
about
this:
https://emea01.safelinks.protection.outlook.com/?url=http%3A%2F%2Fgit
.d
pdk.org%2Fdpdk%2Ftree%2Flib%2Flibrte_eal%2Fcommon%2Fmalloc_heap.c
%23n1169&data=02%7C01%7Cshahafs%40mellanox.com%7Ca1eb06c5a
d794e4e725d08d64a537f23%7Ca652971c7d2e4d9ba6a4d149256f461b%7C0%7
C0%7C636778119822376211&sdata=duMis6fW2CTmyRqdOAmlZl5cezCfJ
aeuoo61QGIBIGk%3D&reserved=0
This will trigger all mem event callbacks, including VFIO DMA mapping
callback.
I see, so I am indeed confused 😊.
On which cases the application should call the existing rte_vfio_dma_map?
If the memory is already mapped and the only way to work with it is through
the rte_malloc mechanism.
I have to be honest, I didn't consider this question before :D I guess there
could be cases where using rte_malloc might not be suitable because it
wastes some memory on malloc elements, i.e. if you want to use N pages as
memory, you'd have to allocate N+1 pages. If memory is at a premium,
maybe manual management of it would be better in some cases.
I had similar thoughts, more related to the usability from the user side.
When application allocated allocates external memory it just wants to use it
for DMA, i.e. put it as the mbuf buf_addr or to populate it w/ a mempool.
It is an "overhead" to create a socket for this external memory, to populate it
w/ the memory, and later on to malloc from this socket (or use the socket id for the
mempool creation).
Not to mention the fact that maybe the application wants to manage this memory
differently than how rte_malloc does.
On the other hand, mapping memory to device before using it for dma is far more
intuitive.
