netdev - Re: [PATCH net-next V4 0/5] vhost: accelerate metadata access through vmap()

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Date:   Tue, 29 Jan 2019 10:34:46 +0800
From:   Jason Wang <jasowang@...hat.com>
To:     "Michael S. Tsirkin" <mst@...hat.com>,
        David Miller <davem@...emloft.net>
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 0/5] vhost: accelerate metadata access through
 vmap()

On 2019/1/27 上午8:31, Michael S. Tsirkin wrote:
> On Sat, Jan 26, 2019 at 02:37:08PM -0800, David Miller wrote:
>> From: Jason Wang <jasowang@...hat.com>
>> Date: Wed, 23 Jan 2019 17:55:52 +0800
>>
>>> 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.
>>>
>>> Test shows about 24% improvement on TX PPS. It should benefit other
>>> cases as well.
>> I've read over the discussion of patch #5 a few times.
>>
>> And it seems to me that, at a minimum, a few things still need to
>> be resolved:
>>
>> 1) More perf data added to commit message.

Ok.

>>
>> 2) Whether invalidate_range_start() and invalidate_range_end() must
>>     be paired.

The reason that vhost doesn't need an invalidate_range_end() is because 
we have a fallback to copy_to_user() friends. So there's no requirement 
to setup the mapping in range_end() or lock the vq between range_start() 
and range_end(). We try to delay the setup of vmap until it will be 
really used in vhost_meta_prefetch() and we hold mmap_sem when trying to 
setup vmap, this will guarantee there's no intermediate state at this time.

>
> Add dirty tracking.

I think this could be solved by introducing e.g 
vhost_meta_prefetch_done() at the end of handle_tx()/handle_rx() and 
call set_page_dirty() for used pages instead of the tricks of 
classifying VMA. (As I saw hugetlbfs has its own set dirty method).

Thanks

>
>> Etc.  So I am marking this series "Changes Requested".