From mboxrd@z Thu Jan  1 00:00:00 1970
From: Anshuman Khandual <khandual@linux.vnet.ibm.com>
Subject: Re: [RFC 0/4] Virtio uses DMA API for all devices
Date: Mon, 6 Aug 2018 14:32:28 +0530
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To: "Michael S. Tsirkin" <mst@redhat.com>, Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: robh@kernel.org, srikar@linux.vnet.ibm.com, linuxram@us.ibm.com, linux-kernel@vger.kernel.org, virtualization@lists.linux-foundation.org, hch@infradead.org, paulus@samba.org, joe@perches.com, linuxppc-dev@lists.ozlabs.org, elfring@users.sourceforge.net, haren@linux.vnet.ibm.com, david@gibson.dropbear.id.au
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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.