hw/scsi/lsi53c895a: Do not abort when DMA requested and no data queued

If asked for DMA request and no data is available, simply wait
for data to be queued, do not abort. This fixes:

  $ cat << EOF | \
    qemu-system-i386 -nographic -M q35,accel=qtest -serial none \
      -monitor none -qtest stdio -trace lsi* \
      -drive if=none,id=drive0,file=null-co://,file.read-zeroes=on,format=raw \
      -device lsi53c895a,id=scsi0 -device scsi-hd,drive=drive0,bus=scsi0.0,channel=0,scsi-id=0,lun=0
  lsi_reset Reset
  lsi_reg_write Write reg DSP2 0x2e = 0xff
  lsi_reg_write Write reg DSP3 0x2f = 0xff
  lsi_execute_script SCRIPTS dsp=0xffff0000 opcode 0x184a3900 arg 0x4a8b2d75
  qemu-system-i386: hw/scsi/lsi53c895a.c:624: lsi_do_dma: Assertion `s->current' failed.

  (gdb) bt
  #5  0x00007ffff4e8a3a6 in __GI___assert_fail
      (assertion=0x5555560accbc "s->current", file=0x5555560acc28 "hw/scsi/lsi53c895a.c", line=624, function=0x5555560adb18 "lsi_do_dma") at assert.c:101
  #6  0x0000555555aa33b9 in lsi_do_dma (s=0x555557805ac0, out=1) at hw/scsi/lsi53c895a.c:624
  #7  0x0000555555aa5042 in lsi_execute_script (s=0x555557805ac0) at hw/scsi/lsi53c895a.c:1250
  #8  0x0000555555aa757a in lsi_reg_writeb (s=0x555557805ac0, offset=47, val=255 '\377') at hw/scsi/lsi53c895a.c:1984
  #9  0x0000555555aa875b in lsi_mmio_write (opaque=0x555557805ac0, addr=47, val=255, size=1) at hw/scsi/lsi53c895a.c:2095

Cc: qemu-stable@nongnu.org
Cc: Gerd Hoffmann <kraxel@redhat.com>
Cc: Vadim Rozenfeld <vrozenfe@redhat.com>
Cc: Stefan Hajnoczi <stefanha@redhat.com>
Reported-by: Jérôme Poulin <jeromepoulin@gmail.com>
Reported-by: Ruhr-University <bugs-syssec@rub.de>
Reported-by: Gaoning Pan <pgn@zju.edu.cn>
Reported-by: Cheolwoo Myung <cwmyung@snu.ac.kr>
Fixes: b96a0da06b ("lsi: move dma_len+dma_buf into lsi_request")
BugLink: https://bugs.launchpad.net/qemu/+bug/697510
BugLink: https://bugs.launchpad.net/qemu/+bug/1905521
BugLink: https://bugs.launchpad.net/qemu/+bug/1908515
Resolves: https://gitlab.com/qemu-project/qemu/-/issues/84
Resolves: https://gitlab.com/qemu-project/qemu/-/issues/305
Resolves: https://gitlab.com/qemu-project/qemu/-/issues/552
Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Laurent Vivier <lvivier@redhat.com>
Message-Id: <20211123111732.83137-2-philmd@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

hw/scsi/lsi53c895a.c

@@ -621,8 +621,7 @@ static void lsi_do_dma(LSIState *s, int out)
     dma_addr_t addr;
     SCSIDevice *dev;
 
-    assert(s->current);
-    if (!s->current->dma_len) {
+    if (!s->current || !s->current->dma_len) {
         /* Wait until data is available.  */
         trace_lsi_do_dma_unavailable();
         return;