On 18-Sep-18 1:29 PM, Shreyansh Jain wrote:
On Monday 17 September 2018 06:30 PM, Burakov, Anatoly wrote:
On 17-Sep-18 1:16 PM, Shahaf Shuler wrote:
Monday, September 17, 2018 1:07 PM, Burakov, Anatoly:
Subject: Re: [dpdk-dev] [PATCH 00/16] Support externally allocated
memory
in DPDK
On 13-Sep-18 8:44 AM, Shahaf Shuler wrote:
[...]
The responsibility to ensure memory is accessible before using it is
on the shoulders of the user - there is no checking done with regards
to validity of the memory (nor could there be...).
That makes sense. However who should be in-charge of mapping this
memory for dma access?
The user or internally be the PMD when encounter the first packet
or while
traversing the existing mempools?
Hi Shahaf,
There are two ways this can be solved. The first way is to perform VFIO
mapping automatically on adding/attaching memory. The second is to
force
user to do it manually. For now, the latter is chosen because user
knows best
if they intend to do DMA on that memory, but i'm open to suggestions.
I agree with that approach, and will add not only if the mempool is
for dma or not but also which ports will use this mempool (this can
effect on the mapping).
That is perhaps too hardware-specific - this should probably be
handled inside the driver callbacks.
However I don't think this is generic enough to use only VFIO. As you
said, there are some devices not using VFIO for mapping rather some
proprietary driver utility.
IMO DPDK should introduce generic and device agnostic APIs to the user.
My suggestion is instead of doing vfio_dma_map that or vfio_dma_unmap
that have a generic dma_map(uint8_t port, address, len). Each driver
will register with its own mapping callback (can be vfio_dma_map).
It can be outside of this series, just wondering the people opinion
on such approach.
I don't disagree. I don't like bus/net/etc drivers doing their own
thing with regards to mapping, and i would by far prefer generic way
to set up DMA maps, to which VFIO will be a subscriber.
There is an issue with some devices and buses (i.e. bus/fslmc)
bypassing EAL
VFIO infrastructure and performing their own VFIO/DMA mapping magic,
but
solving that problem is outside the scope of this patchset. Those
devices/buses should fix themselves :)
DMA mapping is a very common principle and can be easily be a candidate
for lets-make-generic-movement, but, being close to hardware (or
hardware specific), it does require the driver to have some flexibility
in terms of its eventual implementation.
Perhaps i didn't word my response clearly enough. I didn't mean to say
(or imply) that EAL must handle all DMA mappings itself. Rather, EAL
should provide a generic infrastructure of maintaining current mappings
etc., and provide a subscription mechanism for other users (e.g.
drivers) so that the details of implementation of exactly how to map
things for DMA is up to the drivers.
In other words, we agree :)
I maintain one of those drivers (bus/fslmc) in DPDK which needs to have
special VFIO layer - and from that experience, I can say that VFIO
mapping does require some flexibility. SoC semantics are sometimes too
complex to pin to general-universally-agreed-standard concept. (or, one
can easily call it a 'bug', while it is a 'feature' for others :D)
In fact, NXP has another driver (bus/dpaa) which doesn't even work with
VFIO - loves to work directly with Phys_addr. And, it is not at a lower
priority than one with VFIO.
Thus, I really don't think a strongly controlled VFIO mapping should be
EAL's responsibility. Failure because of lack of mapping is a driver's
problem.
While EAL doesn't necessarily need to be involved with mapping things
for VFIO, i believe it does need to be the authority on what gets
mapped. The user needs a way to make arbitrary memory available for DMA
- this is where EAL comes in. VFIO itself can be factored out into a
separate subsystem (DMA drivers, anyone? :D ), but given that memory
cometh and goeth (external memory included), and given that some things
tend to be a bit complicated [*], EAL needs to know when something is
supposed to be mapped or unmapped, and when to notify subscribers that
they may have to refresh their DMA maps.
[*] for example, VFIO can only do mappings whenever there are devices
actually attached to a VFIO container, so we have to maintain all maps
between hotplug events to ensure that memory set up for DMA doesn't
silently get unmapped on device detach and subsequent attach.
--
Thanks,
Anatoly
