On Tue, Jun 08, 2021 at 09:31:28PM -0700, Jiang Wang . wrote:
On Tue, May 18, 2021 at 9:59 PM Jiang Wang . <jiang.wang@xxxxxxxxxxxxx> wrote:
On Tue, May 18, 2021 at 6:02 AM Stefano Garzarella <sgarzare@xxxxxxxxxx> wrote:
>
> On Mon, May 17, 2021 at 11:33:06PM -0700, Jiang Wang . wrote:
> >On Mon, May 17, 2021 at 4:02 AM Stefano Garzarella <sgarzare@xxxxxxxxxx> wrote:
> >>
> >> On Fri, May 14, 2021 at 11:55:29AM -0700, Jiang Wang . wrote:
> >> >On Fri, May 14, 2021 at 8:17 AM Stefano Garzarella <sgarzare@xxxxxxxxxx> wrote:
> >> >> On Thu, May 13, 2021 at 04:26:03PM -0700, Jiang Wang . wrote:
> >>
> >> [...]
> >>
> >> >> >I see. I will add some limit to dgram packets. Also, when the
> >> >> >virtqueues
> >> >> >are shared between stream and dgram, both of them need to grab a lock
> >> >> >before using the virtqueue, so one will not completely block another one.
> >> >>
> >> >> I'm not worried about the concurrent access that we definitely need to
> >> >> handle with a lock, but more about the uncontrolled packet sending that
> >> >> dgram might have, flooding the queues and preventing others from
> >> >> communicating.
> >> >
> >> >That is a valid concern. Let me explain how I would handle that if we
> >> >don't add two new virtqueues. For dgram, I also add a dgram_send_pkt_list,
> >> >which is similar to send_pkt_list for stream (and seqpacket). But there
> >> >is one difference. The dgram_send_pkt_list has a maximum size setting,
> >> >and keep tracking how many pkts are in the list. The track number
> >> >(dgram_send_pkt_list_size) is increased when a packet is added
> >> >to the list and is decreased when a packet
> >> >is removed from the list and added to the virtqueue. In
> >> >virtio_transport_send_pkt, if the current
> >> >dgram_send_pkt_list_size is equal
> >> >to the maximum ( let's say 128), then it will not add to the
> >> >dgram_send_pkt_list and return an error to the application.
> >>
> >> For stream socket, we have the send_pkt_list and the send worker because
> >> the virtqueue can be full and the transmitter needs to wait available
> >> slots, because we can't discard packets.
> >> For dgram I think we don't need this, so we can avoid the
> >> dgram_send_pkt_list and directly enqueue packets in the virtqueue.
> >>
For the question of whether we need dgram_send_pkt_list, I tried to remove
it and that has no problem with virtio vsock in the guest. But on the host, we
still need to keep the dgram_send_pkt_list. The reason is that we cannot
access virtqueue memory reliably in the syscall handling of an
arbitrary process.
The virtqueue memory is in the QEMU process virtual memory and may be
paged out.
I see, I think in that case we can use the virtqueue size as limit for
the dgram_send_pkt_list.
I mean for example if the virtqueue has 128 elements, we can queue at
least 128 packets in the dgram_send_pkt_list.
If you have a better idea go ahead, we can discuss this implementation
detail in the RFC linux series :-)
Thanks,
Stefano
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