[PATCH v2 2/8] net/e1000e: Permit disabling interrupt throttling

[Nicholas Piggin]

The spec explicitly permits xITR register interval field to have a value
of zero to disable throttling. The e1000e model already allows for this
in the throttling logic, so remove the minimum value for the register.

The spec appears to say there is a maximum observable interrupt rate
when throttling is enabled, regardless of ITR value, so throttle timer
calculation is clamped to that minimum value.

EITR registers default to 0, as specified in spec 7.4.4.

Signed-off-by: Nicholas Piggin <npig...@gmail.com>
---
 hw/net/e1000e_core.c | 25 +++++++++++++++++--------
 1 file changed, 17 insertions(+), 8 deletions(-)

diff --git a/hw/net/e1000e_core.c b/hw/net/e1000e_core.c
index 24138587905..96f74f1ea14 100644
--- a/hw/net/e1000e_core.c
+++ b/hw/net/e1000e_core.c
@@ -51,8 +51,17 @@
 
 #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.
+ */
+#define E1000E_DEFAULT_ITR (500)
+
+/*
+ * spec 7.4.4 ITR rules says the maximum observable interrupt rate from the
+ * adapter should not exceed 7813/s (corresponding to 500).
+ */
+#define E1000E_EFFECTIVE_MIN_XITR (500)
 
 #define E1000E_MAX_TX_FRAGS (64)
 
@@ -105,8 +114,9 @@ e1000e_lower_legacy_irq(E1000ECore *core)
 static inline void
 e1000e_intrmgr_rearm_timer(E1000IntrDelayTimer *timer)
 {
-    int64_t delay_ns = (int64_t) timer->core->mac[timer->delay_reg] *
-                                 timer->delay_resolution_ns;
+    uint32_t delay = MAX(timer->core->mac[timer->delay_reg],
+                         E1000E_EFFECTIVE_MIN_XITR);
+    int64_t delay_ns = (int64_t)delay * timer->delay_resolution_ns;
 
     trace_e1000e_irq_rearm_timer(timer->delay_reg << 2, delay_ns);
 
@@ -2783,7 +2793,7 @@ e1000e_set_itr(E1000ECore *core, int index, uint32_t val)
     trace_e1000e_irq_itr_set(val);
 
     core->itr_guest_value = interval;
-    core->mac[index] = MAX(interval, E1000E_MIN_XITR);
+    core->mac[index] = interval;
 }
 
 static void
@@ -2795,7 +2805,7 @@ 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;
-    core->mac[index] = MAX(interval, E1000E_MIN_XITR);
+    core->mac[index] = interval;
 }
 
 static void
@@ -3444,8 +3454,7 @@ 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)
-- 
2.47.1