linux-kernel - Re: [RFC PATCH V3 0/5] Hi:

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Message-ID: <c15a17b3-b604-a105-af44-a7e6dfd79801@redhat.com>
Date:   Mon, 7 Jan 2019 11:53:41 +0800
From:   Jason Wang <jasowang@...hat.com>
To:     "Michael S. Tsirkin" <mst@...hat.com>
Cc:     kvm@...r.kernel.org, virtualization@...ts.linux-foundation.org,
        netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
        davem@...emloft.net
Subject: Re: [RFC PATCH V3 0/5] Hi:


On 2019/1/7 上午11:28, Michael S. Tsirkin wrote:
> On Mon, Jan 07, 2019 at 10:19:03AM +0800, Jason Wang wrote:
>> On 2019/1/3 上午4:47, Michael S. Tsirkin wrote:
>>> On Sat, Dec 29, 2018 at 08:46:51PM +0800, Jason Wang wrote:
>>>> This series tries to access virtqueue metadata through kernel virtual
>>>> address instead of copy_user() friends since they had too much
>>>> overheads like checks, spec barriers or even hardware feature
>>>> toggling.
>>> Will review, thanks!
>>> One questions that comes to mind is whether it's all about bypassing
>>> stac/clac.  Could you please include a performance comparison with
>>> nosmap?
>>>
>> On machine without SMAP (Sandy Bridge):
>>
>> Before: 4.8Mpps
>>
>> After: 5.2Mpps
> OK so would you say it's really unsafe versus safe accesses?
> Or would you say it's just a better written code?


It's the effect of removing speculation barrier.


>
>> On machine with SMAP (Broadwell):
>>
>> Before: 5.0Mpps
>>
>> After: 6.1Mpps
>>
>> No smap: 7.5Mpps
>>
>>
>> Thanks
>
> no smap being before or after?
>

Let me clarify:


Before (SMAP on): 5.0Mpps

Before (SMAP off): 7.5Mpps

After (SMAP on): 6.1Mpps


Thanks