linux-kernel - Re: [RFC PATCH V2 5/5] vhost: access vq metadata through kernel virtual address

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Message-ID: <3b00dc4a-b1b7-aa3f-c4ba-515138e4d6dd@redhat.com>
Date:   Tue, 12 Mar 2019 15:15:22 +0800
From:   Jason Wang <jasowang@...hat.com>
To:     "Michael S. Tsirkin" <mst@...hat.com>
Cc:     Andrea Arcangeli <aarcange@...hat.com>, kvm@...r.kernel.org,
        virtualization@...ts.linux-foundation.org, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org, peterx@...hat.com,
        linux-mm@...ck.org, Jerome Glisse <jglisse@...hat.com>
Subject: Re: [RFC PATCH V2 5/5] vhost: access vq metadata through kernel
 virtual address


On 2019/3/12 上午11:50, Michael S. Tsirkin wrote:
>>>> Using direct mapping (I
>>>> guess kernel will always try hugepage for that?) should be better and we can
>>>> even use it for the data transfer not only for the metadata.
>>>>
>>>> Thanks
>>> We can't really. The big issue is get user pages. Doing that on data
>>> path will be slower than copyXuser.
>> I meant if we can find a way to avoid doing gup in datapath. E.g vhost
>> maintain a range tree and add or remove ranges through MMU notifier. Then in
>> datapath, if we find the range, then use direct mapping otherwise
>> copy_to_user().
>>
>> Thanks
> We can try. But I'm not sure there's any reason to think there's any
> locality there.
>

Ok, but what kind of locality do you mean here?

Thanks