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Subject: Re: [PATCH RFC 0/4] vsock/virtio: optimizations to increase the
 throughput
To:     Stefan Hajnoczi <stefanha@redhat.com>
Cc:     Stefano Garzarella <sgarzare@redhat.com>, netdev@vger.kernel.org,
        "Michael S. Tsirkin" <mst@redhat.com>, kvm@vger.kernel.org,
        virtualization@lists.linux-foundation.org,
        linux-kernel@vger.kernel.org,
        "David S. Miller" <davem@davemloft.net>
References: <20190404105838.101559-1-sgarzare@redhat.com>
 <a98d194d-e257-41ff-82e7-de2d16d2f4d4@redhat.com>
 <20190408094416.GQ15001@stefanha-x1.localdomain>
From:   Jason Wang <jasowang@redhat.com>
Message-ID: <1484dbc0-0413-b21a-6a48-be58da9cb722@redhat.com>
Date:   Tue, 9 Apr 2019 16:36:22 +0800
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On 2019/4/8 下午5:44, Stefan Hajnoczi wrote:
> On Mon, Apr 08, 2019 at 02:43:28PM +0800, Jason Wang wrote:
>> Another thing that may help is to implement sendpage(), which will greatly
>> improve the performance.
> I can't find documentation for ->sendpage().  Is the idea that you get a
> struct page for the payload and can do zero-copy tx?


Yes.


>   (And can userspace
> still write to the page, invalidating checksums in the header?)
>
> Stefan


Userspace can still write to the page, but for correctness (e.g in the 
case of SPLICE_F_GIFT describe by vmsplice(2)), it should not do this. 
For vmsplice, it may hard to detect the time to reuse the page. Maybe we 
MSG_ZEROCOPY[1] is better.

Anyway, sendpage() could be still useful for sendfile() or splice().

Thanks

[1] https://netdevconf.org/2.1/papers/netdev.pdf