On Mon, Aug 06, 2018 at 04:36:43PM +0300, Michael S. Tsirkin wrote:

On Mon, Aug 06, 2018 at 02:32:28PM +0530, Anshuman Khandual wrote:

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On 08/05/2018 05:54 AM, Michael S. Tsirkin wrote:

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

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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.

Did we get better results (lower regression due to indirect calls) with
the suggested mitigation ? Just curious.

I'm referring to this:
	I wonder whether we can support map_sg and friends being NULL, then use
	that when mapping is an identity. A conditional branch there is likely
	very cheap.

I don't think anyone tried implementing this yes.

I've done something very similar in the thread I posted a few years
ago. I plan to get a version of that upstream for 4.20, but it won't
cover the virtio case, just the real direct mapping.