Re: [Qemu-devel] [PATCH 0/13] Add generic packet buffering API

[Avi Kivity]

Mark McLoughlin wrote:
> Hi,
>         This patch series works towards two[1] goals:
>
>   1) Merging the code in qemu-kvm which drains the tap file descriptor
>      when it is readable and buffers a packet if it fails to add it to
>      the NIC receive queue
>
>   2) Using the new TAP_SETSNDBUF ioctl() to put a limit on the number
>      of in-flight packets allowed on a tap device; in this case, if a
>      NIC pops a packet from its transmit queue, we need to be able to
>      buffer said packet if the tap queue is full
>   

Isn't it better instead to unpop the buffer?  The NIC tx ring already is 
a buffer, no need to add another one on top.

It will need adjustments to the device models; for example we'll need 
virtqueue_pop_commit() after we're certain the tap had enough room for 
our packet and virtqueue_pop_cancel() (to unmap the buffers) if we don't.

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