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Re: [Qemu-devel] [PATCH v1 0/1] Allow TAP to unnegotiate virtio-net head

From: Jason Wang
Subject: Re: [Qemu-devel] [PATCH v1 0/1] Allow TAP to unnegotiate virtio-net header
Date: Tue, 19 Feb 2019 17:35:02 +0800
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.4.0

On 2019/2/19 下午4:28, Vincenzo Maffione wrote:
Il giorno mar 19 feb 2019 alle ore 07:48 Jason Wang <address@hidden>
ha scritto:

On 2019/2/18 下午7:53, Vincenzo Maffione wrote:
Hi Jason,
    Thanks for the quick reply.

My PV device (to be open sourced soon) uses the QEMU net backend
in a way similar to virtio-net.
For example it uses qemu_set_offload(), qemu_has_vnet_hdr_len(),
qemu_using_vnet_hdr(), qemu_send_packet(), etc.
This means that the device itself does not know which net backend is
of course.
In addition to TAP, also the netmap backend supports the virtio-net
and so the PV device will work with both.
Regarding the virtio-net header, the netmap backend is more flexible
because you can change the virtio-net header length as you wish (choosing
between 0, 10 and 12 bytes).

Interesting, actually, there's case that vnet header is not used even
for virtio-net, e.g for the case of XDP withouth csum support. If we
don't pass vnet header in this case, we may get even higher PPS.

I guess in this case we simply pass a zeroed header. According to my
experience I think it's unlikely that PPS are affected by having 12
additional bytes to copy around, as long as no processing is done on the
header. I may be wrong of course.

The problem is device doesn't know whether it was zeroed, so it need to examine some fields.


My problem cannot be solved by making sure that TAPs are opened without
IFF_VNET_HDR. What I want instead
is to dynamically change the length of the virtio-net header that the
backend accepts, switching between 12 bytes (virtio 1.0 header) and 0 (no
header). By "dynamically" I mean while the guest (and the device) is
running. This is possible and safe if we get rid of that assert().
The virtio-net device does not need that flexibility (once you enable the
vtnet header, you never disable that again), so I guess that's why you
that assert().
My PV device can be reconfigured on the fly in such a way it may or not
be aware of the virtio-net header, and as a consequence it may need to
qemu_using_vnet_hdr() with true or false (multiple times).

Does it make sense?

Yes, it is. But I think maybe it's better to send this patch with your
new PV device implementation.

Sounds good, thanks.



Il giorno lun 18 feb 2019 alle ore 05:19 Jason Wang <address@hidden

ha scritto:

On 2019/2/16 上午1:46, Vincenzo Maffione wrote:
     I was doing experiments with a custom paravirtualized net device,
and I ran into a limitation of the TAP backend. I see from the kernel
code that it is not possible to set the TAP virtio-net header
length to something different from 10 or 12, which means that it
is not possible to set it to 0. That's fine.
The QEMU implementation of the TAP backend already supports
the case where virtio-net header is set to 10 or 12 in kernel,
but the emulated/paravirtualized net device is not using that:
the TAP backend will just strip/prepend a zeroed header in this case
(e.g., see the if statement in tap_receive_iov()).
However, the tap_using_vnet_hdr() has an assert() that prevents
this situation to happen, and I don't understand why. Maybe it
is a leftover? I tried to remove the assert and by doing that
my paravirtualized device was able to stop using the virtio-net

If  I understand this correctly, your PV device is API compatible with
TAP? Then you may just adding code to call TUNSETIFF without

Vincenzo Maffione (1):
     net: tap: allow net frontends to un-negotiate virtio-net header

    net/tap.c | 1 -
    1 file changed, 1 deletion(-)

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