esp-scsi: flush FIFO before sending SCSI command

The ESP FIFO is used as a buffer for DMA requests and so isn't guaranteed to
be empty in the case of SCSI errors or a mixed DMA/non-DMA request. Flush the
FIFO before sending a SCSI command to guarantee that it is correctly
positioned at the start of the FIFO.

Signed-off-by: Mark Cave-Ayland <>
Reviewed-by: Paolo Bonzini <>
Reviewed-by: Philippe Mathieu-Daudé <>
Message-ID: <>
Signed-off-by: Gerd Hoffmann <>
diff --git a/src/hw/esp-scsi.c b/src/hw/esp-scsi.c
index cc25f22..e4815aa 100644
--- a/src/hw/esp-scsi.c
+++ b/src/hw/esp-scsi.c
@@ -46,6 +46,7 @@
 #define ESP_DMA_WMAC  0x58c
 #define ESP_CMD_DMA      0x80
+#define ESP_CMD_FLUSH    0x01
 #define ESP_CMD_RESET    0x02
 #define ESP_CMD_TI       0x10
 #define ESP_CMD_ICCS     0x11
@@ -96,6 +97,9 @@
     outb(target, iobase + ESP_WBUSID);
+    /* Clear FIFO before sending command.  */
+    outb(ESP_CMD_FLUSH, iobase + ESP_CMD);
      * We need to pass the LUN at the beginning of the command, and the FIFO
      * is only 16 bytes, so we cannot support 16-byte CDBs.  The alternative