Benjamin Herrenschmidt
2018-Aug-04 01:21 UTC
[RFC 0/4] Virtio uses DMA API for all devices
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.
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
On 08/05/2018 05:54 AM, Michael S. Tsirkin wrote:> 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.Did we get better results (lower regression due to indirect calls) with the suggested mitigation ? Just curious.