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Date:   Mon, 6 Aug 2018 08:24:06 -0700
From:   Christoph Hellwig <hch@infradead.org>
To:     "Michael S. Tsirkin" <mst@redhat.com>
Cc:     Anshuman Khandual <khandual@linux.vnet.ibm.com>,
        Benjamin Herrenschmidt <benh@kernel.crashing.org>,
        robh@kernel.org, srikar@linux.vnet.ibm.com, aik@ozlabs.ru,
        Jason Wang <jasowang@redhat.com>, linuxram@us.ibm.com,
        linux-kernel@vger.kernel.org,
        virtualization@lists.linux-foundation.org, hch@infradead.org,
        paulus@samba.org, joe@perches.com, david@gibson.dropbear.id.au,
        linuxppc-dev@lists.ozlabs.org, elfring@users.sourceforge.net,
        haren@linux.vnet.ibm.com
Subject: Re: [RFC 0/4] Virtio uses DMA API for all devices
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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:
> > 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.
> 
> 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.