[Qemu-devel] [PATCH] e1000: Delay LSC until mask is active

[Alexander Graf <agraf@suse.de>]

Mac OS X reads ICR on every interrupt. When the IRQ line is shared, this may
result in a race where LSC is not interpreted yet, but already gets cleared.

The guest already has a way of telling us that it can interpret LSC events
though and that's via the interrupt mask register (IMS).

So if we just leave the LSC interrupt bit pending, but invisible to the guest
as long as it's not ready to receive LSC interrupts, we basically defer the
interrupt to the earliest point in time when the guest would know how to
handle it.

This patch fixes occasional e1000 link detection hickups with Mac OS X.

Reported-by: Gabriel Somlo <somlo@cmu.edu>
Signed-off-by: Alexander Graf <agraf@suse.de>
---
 hw/net/e1000.c | 17 ++++++++++++++++-
 1 file changed, 16 insertions(+), 1 deletion(-)

diff --git a/hw/net/e1000.c b/hw/net/e1000.c
index 0fc29a0..595e034 100644
--- a/hw/net/e1000.c
+++ b/hw/net/e1000.c
@@ -1090,9 +1090,24 @@ static uint32_t
 mac_icr_read(E1000State *s, int index)
 {
     uint32_t ret = s->mac_reg[ICR];
+    uint32_t new_icr = 0;
 
     DBGOUT(INTERRUPT, "ICR read: %x\n", ret);
-    set_interrupt_cause(s, 0, 0);
+
+    /*
+     * Mac OS X reads ICR on every interrupt. When the IRQ line is shared,
+     * this may result in a race where LSC is not interpreted yet, but
+     * already gets cleared.
+     *
+     * The easiest fix is to delay LSC events until after they have been
+     * properly unmasked, so let's just claim we never saw any here.
+     */
+    if ((ret & E1000_ICS_LSC) && !(s->mac_reg[IMS] & E1000_ICS_LSC)) {
+        ret &= ~E1000_ICS_LSC;
+        new_icr |= E1000_ICS_LSC;
+    }
+
+    set_interrupt_cause(s, 0, new_icr);
     return ret;
 }
 
-- 
1.8.1.4