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Date:   Sun, 5 Aug 2018 03:24:28 +0300
From:   "Michael S. Tsirkin" <mst@redhat.com>
To:     Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc:     Jason Wang <jasowang@redhat.com>,
        Anshuman Khandual <khandual@linux.vnet.ibm.com>,
        virtualization@lists.linux-foundation.org,
        linux-kernel@vger.kernel.org, linuxppc-dev@lists.ozlabs.org,
        aik@ozlabs.ru, robh@kernel.org, joe@perches.com,
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        mpe@ellerman.id.au, hch@infradead.org, linuxram@us.ibm.com,
        haren@linux.vnet.ibm.com, paulus@samba.org,
        srikar@linux.vnet.ibm.com
Subject: Re: [RFC 0/4] Virtio uses DMA API for all devices
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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