netdev - Re: [PATCH net-next V4 5/5] vhost: access vq metadata through kernel virtual address

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Date:   Fri, 25 Jan 2019 17:16:32 +0800
From:   Jason Wang <jasowang@...hat.com>
To:     "Michael S. Tsirkin" <mst@...hat.com>
Cc:     virtualization@...ts.linux-foundation.org, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org, kvm@...r.kernel.org
Subject: Re: [PATCH net-next V4 5/5] vhost: access vq metadata through kernel
 virtual address


On 2019/1/25 上午11:00, Michael S. Tsirkin wrote:
> On Thu, Jan 24, 2019 at 12:07:54PM +0800, Jason Wang wrote:
>>> Meanwhile, could you pls post data comparing this last patch with the
>>> below?  This removes the speculation barrier replacing it with a
>>> (useless but at least more lightweight) data dependency.
>> SMAP off
>>
>> Your patch: 7.2MPPs
>>
>> vmap: 7.4Mpps
> OK so while we keep looking into vmap, why don't we merge something like
> the below?  Seems quite straight forward ...
>

The problem is it gives ~8% regression on PPS when SMAP is on. This is 
probably because the latency of lfence is hided by previous stac as 
mentioned in the commit b3bbfb3fb5d25776b8e3f361d2eedaabb0b496cd.

Thanks