Re: NVMe peer2peer TLPs over NTB getting split

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On Tue, Apr 30, 2019 at 03:27:53PM -0600, Logan Gunthorpe wrote:
> On 2019-04-24 4:46 p.m., Eric Pilmore wrote:
> > Hi Folks,
> > 
> > Does anybody know why a Host Bridge might break up a full-sized (max
> > payload) TLP into single byte TLPs when those TLPs are traveling from
> > peer-to-peer?
> 
> Host bridges can't be relied on to do the right thing with respect to
> P2P. This is why the p2pdma code explicitly rejects them. Bad
> performance is often the symptom and splitting may be the cause (I've
> never bothered to stick an analyzer on it. There are patches floating
> around to add a whitelist to p2pdma which would be what you'd want to do
> and avoid anything that doesn't go through a switch.

Note that Max Payload Size may not be the same across root ports,
so splitting transactions may be the correct thing to do under some
circumstances. Kernel parameter "pci=pcie_bus_peer2peer" should make
all MPS settings the same, though I doubt that will help for the
hardware desribed here.



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