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SubjectRe: [PATCH net-next 0/3] vhost: accelerate metadata access through vmap()

On 2018/12/25 上午2:12, Michael S. Tsirkin wrote:
> On Mon, Dec 24, 2018 at 04:32:39PM +0800, Jason Wang wrote:
>> On 2018/12/14 下午8:33, Michael S. Tsirkin wrote:
>>> On Fri, Dec 14, 2018 at 11:42:18AM +0800, Jason Wang wrote:
>>>> On 2018/12/13 下午11:27, Michael S. Tsirkin wrote:
>>>>> On Thu, Dec 13, 2018 at 06:10:19PM +0800, Jason Wang wrote:
>>>>>> Hi:
>>>>>> 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.
>>>>> Userspace accesses through remapping tricks and next time there's a need
>>>>> for a new barrier we are left to figure it out by ourselves.
>>>> I don't get here, do you mean spec barriers?
>>> I mean the next barrier people decide to put into userspace
>>> memory accesses.
>>>> It's completely unnecessary for
>>>> vhost which is kernel thread.
>>> It's defence in depth. Take a look at the commit that added them.
>>> And yes quite possibly in most cases we actually have a spec
>>> barrier in the validation phase. If we do let's use the
>>> unsafe variants so they can be found.
>> unsafe variants can only work if you can batch userspace access. This is not
>> necessarily the case for light load.
> Do we care a lot about the light load? How would you benchmark it?

If we can reduce the latency that's will be more than what we expect.

1 byte TCP_RR shows 1.5%-2% improvement.

>>>> And even if you're right, vhost is not the
>>>> only place, there's lots of vmap() based accessing in kernel.
>>> For sure. But if one can get by without get user pages, one
>>> really should. Witness recently uncovered mess with file
>>> backed storage.
>> We only pin metadata pages, I don't believe they will be used by any DMA.
> It doesn't matter really, if you dirty pages behind the MM back
> the problem is there.

Ok, but the usual case is anonymous pages, do we use file backed pages
for user of vhost? And even if we use sometime, according to the pointer
it's not something that can fix, RFC has been posted to solve this issue.


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