RE: [Qemu-devel] patch for ne2000.c

[Han, Zhu]

Any comments for this patch?

Best Regards, 
hanzhu
-----Original Message-----
From: address@hidden [mailto:address@hidden On Behalf Of Han, Zhu
Sent: 2006年5月9日 12:27
To: address@hidden
Subject: [Qemu-devel] patch for ne2000.c

Hi, All!

I'm a developer working on xen project! It's well known that xen has
adopted a lot of codes and features from QEMU, especially the Device
Mode Part!

I fix a bug for ne2000 device emulation code in XEN and I expect it to
be a potential bug for QEMU, either! Because you are all device mode
experts, I submit this patch to you at first in order to ask you to
review my patch. 

Several notes:
1) Because XEN use event driven mechanism in the main_loop(), irq may be
missed due to the rather high speed and large file! For example, the
ne2000_receive will filled up with the buffer and set up the ENISR_RX
signal, however, the driver could ack and clear the ENISR_RX signal due
to it could only handle a certain amount of packets once in it's
interrupt handling routine!  The consequence for this specific steps is
the netcard buffer is full but it never resend the ENISR_RX signal, at
the last, the netcard will be halted! This problem could be rather rare
for QEMU. Anyway, it's a potential bug.
2) Many of the ne2000 spec said we should set boundary register should
be set to indicate the last receive buffer page the host has read, and
the driver in linux follows this guideline. So, we boundary == index,
the buffer for the netcard is full and we can't write any packets into
this buffer. This minor fix could prevent the ne2000 emulated card from
overflow and destroying the previous received packet page! This problem
could also be rare for QEMU since it could happen only under extreme
circumstance! 

Any feedbacks and comments will be appreciated! 

--- qemu-snapshot-2006-05-07_23\hw\ne2000.c     Mon May 08 16:13:49 2006
+++ ./ne2000.c  Mon May 08 16:57:33 2006
@@ -159,9 +159,19 @@
     }
 }
 
+static int ne2000_buffer_full(NE2000State *s);
 static void ne2000_update_irq(NE2000State *s)
 {
     int isr;
+
+    if(ne2000_buffer_full(s)
+            && !(s->isr & ENISR_RX)){
+       /* The freeing space is not enough, tell the ne2k driver
+        * to fetch these packets!
+        */
+        s->isr |= ENISR_RX;
+    }
+    
     isr = (s->isr & s->imr) & 0x7f;
 #if defined(DEBUG_NE2000)
     printf("NE2000: Set IRQ line %d to %d (%02x %02x)\n",
@@ -206,7 +216,10 @@
 
     index = s->curpag << 8;
     boundary = s->boundary << 8;
-    if (index < boundary)
+    if (index <= boundary)
+       /* when index == boundary, we should assume 
+        * the buffer is full instead of empty!
+        */
         avail = boundary - index;
     else
         avail = (s->stop - s->start) - (index - boundary);

Best Regards, 
hanzhu