On Tue, Oct 10, 2017 at 09:56:44AM +0000, Liang, Cunming wrote:
> > DESC_WRAP: used by device to poll. Driver sets it to a *different*
> > value every time it overwrites a descriptor. How to achieve it?
> > since descriptors are written out in ring order, simply maintain the
> > current value internally (start value 1) and flip it every time you
> > overwrite the first descriptor.
> > Device leaves it intact when overwriting a descriptor.
Ok, get it now.
>
> This is confusing me a bit.
>
> My understanding is: 1. the internally kept wrap value only flipped
> when the first descriptor is overwritten
>
> 2. the moment the first descriptor is written the internal wrap value
> is flipped 0->1 or 1->0 and this value is written to every descriptor
> DESC_WRAP until we reach the first descriptor again
That's right, it's also my take.
DESC_WRAP is only used by driver, device does nothing with that flag.
Yes this is what I tried to say. Can you suggest a better wording then?
I'll give it a try.
The term of DESC_WRAP is fine to me.
Couldn't think of a better name either.
> > After writing down this explanation, I think the names aren't great.
> >
> > Let me try an alternative explanation.
> >
> > ---------------
> > A two-bit field, DRIVER_OWNER, signals the buffer ownership.
> > It has 4 possible values:
> > values 0x1, 0x11 are written by driver values 0x0, 0x10 are written
> > by device
>
> The 0x prefix might add to the confusion here. It is really just two
> bits, no?
Ouch. Yes I meant 0b. Thanks!
0b00, 0b10 are written by device?
I suppose device can only clear high bit, can keep low bit no change.
Then the value written by device can be either 0b01 or 0b00, but 0b10 means to set high bit, no?
> > each time driver writes out a descriptor, it must make sure that the
> > high bit in OWNER changes.
> >
> > each time device writes out a descriptor, it must make sure that the
> > high bit in OWNER does not change.
Typo here? It should be "..., it must make sure that the low bit in OWNER does not change."?
For high bit in OWNER, each time devices writes out a descriptor, it must make sure to clear high bit in OWNER.
> >
> > this is exactly the same functionally, DRIVER is high bit and WRAP
> > is the low bit. Does this make things clearer?
>
> So far it makes sense to me.
It sounds good.
So I implemented two ideas in the DPDK prototype code. The code is
very rough and simple. I'll describe again how I understood the ideas.
1. The one discussed in this thread: Adding two flags DESC_DRIVER and DESC_WRAP.
Driver code: keeps an internal wrap value. Every time we cross the ring
boundary at descriptor 0 the wrap value is flipped. For all
descriptors used from this point on we will set the DESC_WRAP flag if it wasn't
set before or the other way round. Driver code looks at the
DESC_DRIVER flag to check if the descriptor is available for it to
use.
Device code: when dequeuing descriptors from the ring it keeps going
until it sees a different value in the DESC_WRAP bit. Device only
checks this bit but doesn't change it. Device clears DESC_DRIVER when
done with descriptor to signal to driver that descriptor can be reused.
https://github.com/jensfr/dpdk/tree/add-driver-and-wrap-flag
2. Driver writes ID of last written descriptor into index field of first
descriptor and turns on a flag DESC_SKIP in the descriptor. (This idea is from Michael)
Driver code: Let's say driver adds 32 descriptors to the ring at once.
It fills in starting at ring position 0 to 31. It will write 31 to the
index field of descriptor 0.
Device code: When dequeueing descriptors from the ring it looks into
the first descriptor. For example, it looks at desc[0] and the index field is 31
instead of 0. In addition to this the flag DESC_SKIP is set. The
device can expect all descriptors in the ring from 0 to 31 to be ready
for it to dequeue and doesn't have to check the DESC_HW flag.
https://github.com/jensfr/dpdk/tree/desc-hw-index-hint
I tried implementing both ideas on top of the DPDK prototype code
(without the DESC_WB code) and ran a quick test with two testpmd
instances, one with a vhost-user interface and the other one with a
virtio-user device.
From a performance point of view I saw no difference between both
implementations.
I understand that DESC_DRIVER/DESC_WRAP would be better for virtio
hardware implementations?
regards,
Jens
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