On Sat, 2015-09-05 at 13:30 -0700, Peter Crosthwaite wrote: > On Fri, Sep 4, 2015 at 10:30 AM, Peter Maydell <peter.mayd...@linaro.org> > wrote: > > On 4 September 2015 at 18:20, Richard Purdie > > <richard.pur...@linuxfoundation.org> wrote: > >> On Fri, 2015-09-04 at 13:43 +0100, Richard Purdie wrote: > >>> On Fri, 2015-09-04 at 12:31 +0100, Peter Maydell wrote: > >>> > On 4 September 2015 at 12:24, Richard Purdie > >>> > <richard.pur...@linuxfoundation.org> wrote: > >>> > > So just based on that, yes, seems that the rx_fifo looks to be > >>> > > overrunning. I can add the asserts but I think it would just confirm > >>> > > this. > >>> > > >>> > Yes, the point of adding assertions is to confirm a hypothesis. > >>> > >>> I've now confirmed that it does indeed trigger the assert in > >>> smc91c111_receive(). > >> > >> I just tried an experiment where I put: > >> > >> if (s->rx_fifo_len >= NUM_PACKETS) > >> return -1; > >> > >> into smc91c111_receive() and my reproducer stops reproducing the > >> problem. > > Does it just stop the crash or does it eliminate the problem > completely with a fully now-working network?
It stops the crash, the network works great. > >> I also noticed can_receive() could also have a check on buffer > >> availability. Would one of these changes be the correct fix here? > > > > The interesting question is why smc91c111_allocate_packet() doesn't > > fail in this situation. We only have NUM_PACKETS worth of storage, > > shared between the tx and rx buffers, so how could we both have > > already filled the rx_fifo and have a spare packet for the allocate > > function to return? > > Maybe this: > > case 5: /* Release. */ > smc91c111_release_packet(s, s->packet_num); > break; > > The guest is able to free an allocated packet without the accompanying > pop of tx/rx fifo. This may suggest some sort of guest error? > > The fix depends on the behaviour of the real hardware. If that MMIO op > is supposed to dequeue the corresponding queue entry then we may need > to patch that logic to do search the queues and dequeue it. Otherwise > we need to find out the genuine length of the rx queue, and clamp it > without something like Richards patch. There are a few other bits and > pieces that suggest the guest can have independent control of the > queues and allocated buffers but i'm confused as to how the rx fifo > length can get up to 10 in any case. I think I have a handle on what is going on. smc91c111_release_packet() changes s->allocated() but not rx_fifo. can_receive() only looks at s->allocated. We can trigger new network packets to arrive from smc91c111_release_packet() which calls qemu_flush_queued_packets() *before* we change rx_fifo and this can loop. The patch below which explicitly orders the qemu_flush_queued_packets() call resolved the test case I was able to reproduce this problem in. So there are three ways to fix this, either can_receive() needs to check both s->allocated() and rx_fifo, or the code is more explicit about when qemu_flush_queued_packets() is called (as per my patch below), or the case 4 where smc91c111_release_packet() and then smc91c111_pop_rx_fifo(s) is called is reversed. I also tested the latter which also works, albeit with more ugly code. The problem is much more reproducible with the assert btw, booting a qemu image with this and hitting the network interface with scp of a few large files is usually enough. So which patch would be preferred? :) Cheers, Richard Index: qemu-2.4.0/hw/net/smc91c111.c =================================================================== --- qemu-2.4.0.orig/hw/net/smc91c111.c +++ qemu-2.4.0/hw/net/smc91c111.c @@ -185,7 +185,6 @@ static void smc91c111_release_packet(smc s->allocated &= ~(1 << packet); if (s->tx_alloc == 0x80) smc91c111_tx_alloc(s); - qemu_flush_queued_packets(qemu_get_queue(s->nic)); } /* Flush the TX FIFO. */ @@ -237,9 +236,11 @@ static void smc91c111_do_tx(smc91c111_st } } #endif - if (s->ctr & CTR_AUTO_RELEASE) + if (s->ctr & CTR_AUTO_RELEASE) { /* Race? */ smc91c111_release_packet(s, packetnum); + qemu_flush_queued_packets(qemu_get_queue(s->nic)); + } else if (s->tx_fifo_done_len < NUM_PACKETS) s->tx_fifo_done[s->tx_fifo_done_len++] = packetnum; qemu_send_packet(qemu_get_queue(s->nic), p, len); @@ -379,9 +380,11 @@ static void smc91c111_writeb(void *opaqu smc91c111_release_packet(s, s->rx_fifo[0]); } smc91c111_pop_rx_fifo(s); + qemu_flush_queued_packets(qemu_get_queue(s->nic)); break; case 5: /* Release. */ smc91c111_release_packet(s, s->packet_num); + qemu_flush_queued_packets(qemu_get_queue(s->nic)); break; case 6: /* Add to TX FIFO. */ smc91c111_queue_tx(s, s->packet_num);