Re: [Qemu-devel] [PATCH v3 03/30] imx_fec: Change queue flushing heuristics

[Peter Maydell]

On 6 November 2017 at 15:47, Andrey Smirnov <address@hidden> 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.
>
> To prevent the problem, change the code to always flush packet queue
> when ENET_RDAR transitions 0 -> ENET_RDAR_RDAR.
>
> Cc: Peter Maydell <address@hidden>
> Cc: Jason Wang <address@hidden>
> Cc: Philippe Mathieu-Daudé <address@hidden>
> Cc: address@hidden
> Cc: address@hidden
> Cc: address@hidden
> Signed-off-by: Andrey Smirnov <address@hidden>
> 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 field isn't needed any more in this version of the patch, I think?

otherwise
Reviewed-by: Peter Maydell <address@hidden>

thanks
-- PMM