On 18 September 2017 at 20:50, Andrey Smirnov <andrew.smir...@gmail.com> wrote:
> In current implementation, packet queue flushing logic seem to suffer
> from a deadlock like scenario if a packet is received by the interface
> before before Rx ring is initialized by Guest's driver. Consider the
> following sequence of events:
>
> 1. A QEMU instance is started against a TAP device on Linux
> host, running Linux guest, e. g., something to the effect
> of:
>
> qemu-system-arm \
> -net nic,model=imx.fec,netdev=lan0 \
> netdev tap,id=lan0,ifname=tap0,script=no,downscript=no \
> ... rest of the arguments ...
>
> 2. Once QEMU starts, but before guest reaches the point where
> FEC deriver is done initializing the HW, Guest, via TAP
> interface, receives a number of multicast MDNS packets from
> Host (not necessarily true for every OS, but it happens at
> least on Fedora 25)
>
> 3. Recieving a packet in such a state results in
> imx_eth_can_receive() returning '0', which in turn causes
> tap_send() to disable corresponding event (tap.c:203)
>
> 4. Once Guest's driver reaches the point where it is ready to
> recieve packets it prepares Rx ring descriptors and writes
> ENET_RDAR_RDAR to ENET_RDAR register to indicate to HW that
> more descriptors are ready. And at this points emulation
> layer does this:
>
> s->regs[index] = ENET_RDAR_RDAR;
> imx_eth_enable_rx(s);
>
> which, combined with:
>
> if (!s->regs[ENET_RDAR]) {
> qemu_flush_queued_packets(qemu_get_queue(s->nic));
> }
>
> results in Rx queue never being flushed and corresponding
> I/O event beign disabled.
>
> Change the code to remember the fact that can_receive callback was
> called before Rx ring was ready and use it to make a decision if
> receive queue needs to be flushed.
>
> Cc: Peter Maydell <peter.mayd...@linaro.org>
> Cc: Jason Wang <jasow...@redhat.com>
> Cc: qemu-devel@nongnu.org
> Cc: qemu-...@nongnu.org
> Cc: yurov...@gmail.com
> Signed-off-by: Andrey Smirnov <andrew.smir...@gmail.com>
> ---
> hw/net/imx_fec.c | 6 ++++--
> include/hw/net/imx_fec.h | 1 +
> 2 files changed, 5 insertions(+), 2 deletions(-)
>
> diff --git a/hw/net/imx_fec.c b/hw/net/imx_fec.c
> index 84085afe09..767402909d 100644
> --- a/hw/net/imx_fec.c
> +++ b/hw/net/imx_fec.c
> @@ -544,8 +544,9 @@ static void imx_eth_enable_rx(IMXFECState *s)
>
> if (rx_ring_full) {
> FEC_PRINTF("RX buffer full\n");
> - } else if (!s->regs[ENET_RDAR]) {
> + } else if (s->needs_flush) {
> qemu_flush_queued_packets(qemu_get_queue(s->nic));
> + s->needs_flush = false;
> }
>
> s->regs[ENET_RDAR] = rx_ring_full ? 0 : ENET_RDAR_RDAR;
> @@ -930,7 +931,8 @@ static int imx_eth_can_receive(NetClientState *nc)
>
> FEC_PRINTF("\n");
>
> - return s->regs[ENET_RDAR] ? 1 : 0;
> + s->needs_flush = !s->regs[ENET_RDAR];
> + return !!s->regs[ENET_RDAR];
> }
>
> static ssize_t imx_fec_receive(NetClientState *nc, const uint8_t *buf,
> diff --git a/include/hw/net/imx_fec.h b/include/hw/net/imx_fec.h
> index 62ad473b05..4bc8f03ec2 100644
> --- a/include/hw/net/imx_fec.h
> +++ b/include/hw/net/imx_fec.h
> @@ -252,6 +252,7 @@ typedef struct IMXFECState {
> uint32_t phy_int_mask;
>
> bool is_fec;
> + bool needs_flush;
> } IMXFECState;
This looks odd -- I don't think you should need extra
state here. Conceptually what you want is:
* in the can_receive callback, test some function of
various bits of device state to decide whether you can
take data
* in the rest of the device, whenever the device state
changes such that you were previously not able to take
data but now you can, call qemu_flush_queued_packets().
You shouldn't need any extra state to do this, you just
need to fix the bug where you have a code path that
flips ENET_RDAR from 0 to 1 without calling flush
(you might for instance have a helper function for
"set ENET_RDAR" that encapsulates setting the state
and arranging that flush is called).
thanks
-- PMM
