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Date:   Fri, 11 Jan 2019 16:59:13 +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 2019/1/8 下午6:12, Jason Wang wrote:
>
> On 2019/1/7 下午10:47, Michael S. Tsirkin wrote:
>> On Mon, Jan 07, 2019 at 02:58:08PM +0800, Jason Wang wrote:
>>> On 2019/1/5 上午5:41, 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.
>>>> I think it's a reasonable approach.
>>>> However I need to look at whether and which mmu notifiers are 
>>>> invoked before
>>>> writeback. Do you know?
>>>
>>> I don't know but just looking at the MMU notifier ops definition, 
>>> there's no
>>> such callback if my understanding is correct.
>>>
>>> Thanks
>> In that case how are you making sure used ring updates are written back?
>> If they aren't guest will crash ...
>
>
> I think this is the writeback issue you mentioned early. I don't do a 
> followup on the pointer but it looks to me some work is ongoing to fix 
> the issue.
>
> I can investigate it more, but it's not something new, consider the 
> case of VFIO.
>
> Thanks


Ok, after some investigation. The GUP + dirty pages issue is not easy to 
be fixed so it may still take a while.

An idea is switch back to copy_user() friends if we find the metadata 
page is not anonymous.

Does this sound good to you?

Thanks

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