When rtl8139 card is running in standard mode, it is very easy
to overlflow and the receive buffer and get into a siutation
where all packets are dropped. Simply reproduction case is
to ping the guest from the host with 6500 byte packets.
There are actually 2 problems here.
1) When the rtl8129 buffer is overflow, the card emulation
returns the size of the packet back to queue transmission.
This signals successful reception even though the packet
has been dropped. The proper solution is to return 0, so
that the packet is re-queued and will be resubmitted later.
2) When packets are sized such that the fragments end up completely
filling the receive buffer without overflow, the device thinks
that the buffer is actually empty (instead of full). This causes
next packet to over-write the existing packets. With the above
ping reproducer, ever ICMP packet fills the buffer and thus keeps
overwriting the previous packet and never waking up the guest.
The solution here is track the number of unread bytes separately
so we would know if we have anything in buffer to read or not.
V3: Fix the second patch to correctly track unread and available buffer
speace. Prior version used calculation for availble space to track
unread space which was wrong.
V2: instead of tracking buffer_full condition, changed the code, as
suggested by Stefan Hajnoczi, to track the number of unread bytes
instead. We initialize it to 0 at the start, adjust it on every
receive from the network and read from the guest and can set
the number of unread of bytes to full buffer size when the buffer
full.
Vladislav Yasevich (2):
rtl8139: Do not consume the packet during overflow in standard mode.
rtl8139: correctly track full receive buffer in standard mode
hw/net/rtl8139.c | 47 ++++++++++++++++++++++++++++++++++++++++++-----
1 file changed, 42 insertions(+), 5 deletions(-)
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1.9.3
