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

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Message-Id: <74a1e1b8-81e0-84db-6d0d-d8bd9caebb4a@linux.vnet.ibm.com>
Date:   Mon, 6 Aug 2018 14:32:28 +0530
From:   Anshuman Khandual <khandual@...ux.vnet.ibm.com>
To:     "Michael S. Tsirkin" <mst@...hat.com>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>
Cc:     robh@...nel.org, srikar@...ux.vnet.ibm.com, aik@...abs.ru,
        Jason Wang <jasowang@...hat.com>, linuxram@...ibm.com,
        linux-kernel@...r.kernel.org,
        virtualization@...ts.linux-foundation.org, hch@...radead.org,
        paulus@...ba.org, joe@...ches.com, david@...son.dropbear.id.au,
        linuxppc-dev@...ts.ozlabs.org, elfring@...rs.sourceforge.net,
        haren@...ux.vnet.ibm.com
Subject: Re: [RFC 0/4] Virtio uses DMA API for all devices

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.