Re: [RFC 0/4] Virtio uses DMA API for all devices

[Michael S. Tsirkin]

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:
> > > > > 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