On Fri, Aug 03, 2018 at 08:21:26PM -0500, Benjamin Herrenschmidt wrote:

On Fri, 2018-08-03 at 22:08 +0300, Michael S. Tsirkin wrote:

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Please go through these patches and review whether this approach broadly
makes sense. I will appreciate suggestions, inputs, comments regarding
the patches or the approach in general. Thank you.

Jason did some work on profiling this. Unfortunately he reports
about 4% extra overhead from this switch on x86 with no vIOMMU.

The test is rather simple, just run pktgen (pktgen_sample01_simple.sh) in
guest and measure PPS on tap on host.

Thanks

Could you supply host configuration involved please?

I wonder how much of that could be caused by Spectre mitigations
blowing up indirect function calls...

Cheers,
Ben.

I won't be surprised. If yes I suggested a way to mitigate the overhead.

-- 
MSR