Re: XDP performance regression due to CONFIG_RETPOLINE Spectre V2

[Jesper Dangaard Brouer <brouer@redhat.com>]

On Thu, 12 Apr 2018 16:56:53 +0200 Christoph Hellwig <hch@lst.de> wrote:

> On Thu, Apr 12, 2018 at 04:51:23PM +0200, Christoph Hellwig wrote:
> > On Thu, Apr 12, 2018 at 03:50:29PM +0200, Jesper Dangaard Brouer wrote:  
> > > ---------------
> > > Implement support for keeping the DMA mapping through the XDP return
> > > call, to remove RX map/unmap calls.  Implement bulking for XDP
> > > ndo_xdp_xmit and XDP return frame API.  Bulking allows to perform DMA
> > > bulking via scatter-gatter DMA calls, XDP TX need it for DMA
> > > map+unmap. The driver RX DMA-sync (to CPU) per packet calls are harder
> > > to mitigate (via bulk technique). Ask DMA maintainer for a common
> > > case direct call for swiotlb DMA sync call ;-)  
> > 
> > Why do you even end up in swiotlb code?  Once you bounce buffer your
> > performance is toast anyway..  
> 
> I guess that is because x86 selects it as the default as soon as
> we have more than 4G memory. 

I were also confused why I ended up using SWIOTLB (SoftWare IO-TLB),
that might explain it. And I'm not hitting the bounce-buffer case.

How do I control which DMA engine I use? (So, I can play a little)


> That should be solveable fairly easily with the per-device dma ops,
> though.

I didn't understand this part.

I wanted to ask your opinion, on a hackish idea I have...
Which is howto detect, if I can reuse the RX-DMA map address, for TX-DMA
operation on another device (still/only calling sync_single_for_device).

With XDP_REDIRECT we are redirecting between net_device's. Usually
we keep the RX-DMA mapping as we recycle the page. On the redirect to
TX-device (via ndo_xdp_xmit) we do a new DMA map+unmap for TX.  The
question is how to avoid this mapping(?).  In some cases, with some DMA
engines (or lack of) I guess the DMA address is actually the same as
the RX-DMA mapping dma_addr_t already known, right?  For those cases,
would it be possible to just (re)use that address for TX?

-- 
Best regards,
  Jesper Dangaard Brouer
  MSc.CS, Principal Kernel Engineer at Red Hat
  LinkedIn: http://www.linkedin.com/in/brouer