[Qemu-devel] [PATCH] e1000: make ICS write-only

["Michael S. Tsirkin" <mst@redhat.com>]

Since commit b1332393cdd7d023de8f1f8aa136ee7866a18968,
qemu started updating ICS register when interrupt
is sent, with the intent to match spec better
(guests do not actually read this register).
However, the function set_interrupt_cause where ICS
is updated is often called internally by
device emulation so reading it does not produce the last value
written by driver.  Looking closer at the spec,
it documents ICS as write-only, so there's no need
to update it at all. I conclude that while harmless this line is useless
code so removing it is a bit cleaner than keeping it in.

Tested with windows and linux guests.

Cc: Bill Paul <wpaul@windriver.com>
Reported-by: Yan Vugenfirer <yan@daynix.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
---
 hw/e1000.c | 1 -
 1 file changed, 1 deletion(-)

diff --git a/hw/e1000.c b/hw/e1000.c
index 92fb00a..928d804 100644
--- a/hw/e1000.c
+++ b/hw/e1000.c
@@ -230,7 +230,6 @@ set_interrupt_cause(E1000State *s, int index, uint32_t val)
         val |= E1000_ICR_INT_ASSERTED;
     }
     s->mac_reg[ICR] = val;
-    s->mac_reg[ICS] = val;
     qemu_set_irq(s->dev.irq[0], (s->mac_reg[IMS] & s->mac_reg[ICR]) != 0);
 }
 
-- 
MST