[PATCH 8/9] net/e1000e: Fix xITR minimum value

[Nicholas Piggin <npiggin@gmail.com>]

The ITR minimum value may be a mis-reading or ambiguity in the spec.
Section 10.2.4.2 says the maximum observable interrupt rate should never
exceed 7813, but that is in context of example of the interval being
programmed to 500. On the other hand 7.4.4 does say ITR rules permit
no more than that rate.

There is no minimum value specified, and zero is explicitly allowed and
disables throttling logic (which is already supported behaviour in the
throttling code of the models). This seems to fall outside ITR rules, so
should not cause any limit.

Spec 7.4.4 also says that EITR registers should be initialised to zero.

Remove the minimum value from the ITR and EITR registers, and set ITR
default to 500.

Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
---
 hw/net/e1000e_core.c | 24 ++++++++++--------------
 1 file changed, 10 insertions(+), 14 deletions(-)

diff --git a/hw/net/e1000e_core.c b/hw/net/e1000e_core.c
index c5be20bcbbe..34bb5f8096b 100644
--- a/hw/net/e1000e_core.c
+++ b/hw/net/e1000e_core.c
@@ -51,8 +51,13 @@
 
 #include "trace.h"
 
-/* No more then 7813 interrupts per second according to spec 10.2.4.2 */
-#define E1000E_MIN_XITR     (500)
+/*
+ * A suggested range for ITR is 651-5580, according to spec 10.2.4.2, but
+ * QEMU has traditionally set 500 here and spec 7.4.4 ITR rules says the
+ * max observable interrupts from the adapter should be 7813/s (corresponding
+ * to 500).
+ */
+#define E1000E_DEFAULT_ITR (500)
 
 #define E1000E_MAX_TX_FRAGS (64)
 
@@ -2831,11 +2836,7 @@ e1000e_set_itr(E1000ECore *core, int index, uint32_t val)
     trace_e1000e_irq_itr_set(val);
 
     core->itr_guest_value = interval;
-    if (interval == 0) {
-        core->mac[index] = 0;
-    } else {
-        core->mac[index] = MAX(interval, E1000E_MIN_XITR);
-    }
+    core->mac[index] = interval;
 }
 
 static void
@@ -2847,11 +2848,7 @@ e1000e_set_eitr(E1000ECore *core, int index, uint32_t val)
     trace_e1000e_irq_eitr_set(eitr_num, val);
 
     core->eitr_guest_value[eitr_num] = interval;
-    if (interval == 0) {
-        core->mac[index] = 0;
-    } else {
-        core->mac[index] = MAX(interval, E1000E_MIN_XITR);
-    }
+    core->mac[index] = interval;
 }
 
 static void
@@ -3500,8 +3497,7 @@ static const uint32_t e1000e_mac_reg_init[] = {
     [FACTPS]        = E1000_FACTPS_LAN0_ON | 0x20000000,
     [SWSM]          = 1,
     [RXCSUM]        = E1000_RXCSUM_IPOFLD | E1000_RXCSUM_TUOFLD,
-    [ITR]           = E1000E_MIN_XITR,
-    [EITR...EITR + E1000E_MSIX_VEC_NUM - 1] = E1000E_MIN_XITR,
+    [ITR]           = E1000E_DEFAULT_ITR,
 };
 
 static void e1000e_reset(E1000ECore *core, bool sw)
-- 
2.45.2