[Qemu-devel] [PATCH] ehci: Use uframe precision for interrupt threshold checking (v2)

[Hans de Goede]

Before this patch, the following could happen:
1) Transfer completes, raises interrupt
2) .5 ms later we check if the guest has queued up any new transfers
3) We find and execute a new transfer
4) .2 ms later the new transfer completes
5) We re-run our frame_timer to write back the completion, but less then
   1 ms has passed since our last run, so frindex is not changed, so the
   interrupt threshold code delays the interrupt
6) 1 ms from the re-run our frame-timer runs again and finally delivers
   the interrupt

This leads to unnecessary large delays of interrupts, this code fixes this
by changing frindex to uframe precision and using that for interrupt threshold
control, making the interrupt fire at step 5 for guest which have low interrupt
threshold settings (like Linux).

Note that the guest still sees the frindex move in steps of 8 for migration
compatibility.

This boosts Linux read speed of a simple cheap USB thumb drive by 6 %.

Changes in v2:
-Make the guest see frindex move in steps of 8 by modifying ehci_opreg_read,
 rather then using a shadow variable

Signed-off-by: Hans de Goede <address@hidden>
---
 hw/usb/hcd-ehci.c | 70 +++++++++++++++++++++++++++++++++++++------------------
 1 file changed, 47 insertions(+), 23 deletions(-)

diff --git a/hw/usb/hcd-ehci.c b/hw/usb/hcd-ehci.c
index c7a9a7c..320b7e7 100644
--- a/hw/usb/hcd-ehci.c
+++ b/hw/usb/hcd-ehci.c
@@ -109,12 +109,13 @@
 
 #define FRAME_TIMER_FREQ 1000
 #define FRAME_TIMER_NS   (1000000000 / FRAME_TIMER_FREQ)
+#define UFRAME_TIMER_NS  (FRAME_TIMER_NS / 8)
 
 #define NB_MAXINTRATE    8        // Max rate at which controller issues ints
 #define BUFF_SIZE        5*4096   // Max bytes to transfer per transaction
 #define MAX_QH           100      // Max allowable queue heads in a chain
-#define MIN_FR_PER_TICK  3        // Min frames to process when catching up
-#define PERIODIC_ACTIVE  64
+#define MIN_UFR_PER_TICK 24       /* Min frames to process when catching up */
+#define PERIODIC_ACTIVE  512      /* Micro-frames */
 
 /*  Internal periodic / asynchronous schedule state machine states
  */
@@ -967,7 +968,15 @@ static uint64_t ehci_opreg_read(void *ptr, hwaddr addr,
     EHCIState *s = ptr;
     uint32_t val;
 
-    val = s->opreg[addr >> 2];
+    switch (addr) {
+    case FRINDEX:
+        /* Round down to mult of 8, else it can go backwards on migration */
+        val = s->frindex & ~7;
+        break;
+    default:
+        val = s->opreg[addr >> 2];
+    }
+
     trace_usb_ehci_opreg_read(addr + s->opregbase, addr2str(addr), val);
     return val;
 }
@@ -1118,7 +1127,8 @@ static void ehci_opreg_write(void *ptr, hwaddr addr,
         break;
 
     case FRINDEX:
-        val &= 0x00003ff8; /* frindex is 14bits and always a multiple of 8 */
+        val &= 0x00003fff; /* frindex is 14bits */
+        s->usbsts_frindex = val;
         break;
 
     case CONFIGFLAG:
@@ -2237,16 +2247,16 @@ static void ehci_advance_periodic_state(EHCIState *ehci)
     }
 }
 
-static void ehci_update_frindex(EHCIState *ehci, int frames)
+static void ehci_update_frindex(EHCIState *ehci, int uframes)
 {
     int i;
 
-    if (!ehci_enabled(ehci)) {
+    if (!ehci_enabled(ehci) && ehci->pstate == EST_INACTIVE) {
         return;
     }
 
-    for (i = 0; i < frames; i++) {
-        ehci->frindex += 8;
+    for (i = 0; i < uframes; i++) {
+        ehci->frindex++;
 
         if (ehci->frindex == 0x00002000) {
             ehci_raise_irq(ehci, USBSTS_FLR);
@@ -2270,33 +2280,33 @@ static void ehci_frame_timer(void *opaque)
     int need_timer = 0;
     int64_t expire_time, t_now;
     uint64_t ns_elapsed;
-    int frames, skipped_frames;
+    int uframes, skipped_uframes;
     int i;
 
     t_now = qemu_get_clock_ns(vm_clock);
     ns_elapsed = t_now - ehci->last_run_ns;
-    frames = ns_elapsed / FRAME_TIMER_NS;
+    uframes = ns_elapsed / UFRAME_TIMER_NS;
 
     if (ehci_periodic_enabled(ehci) || ehci->pstate != EST_INACTIVE) {
         need_timer++;
 
-        if (frames > ehci->maxframes) {
-            skipped_frames = frames - ehci->maxframes;
-            ehci_update_frindex(ehci, skipped_frames);
-            ehci->last_run_ns += FRAME_TIMER_NS * skipped_frames;
-            frames -= skipped_frames;
-            DPRINTF("WARNING - EHCI skipped %d frames\n", skipped_frames);
+        if (uframes > (ehci->maxframes * 8)) {
+            skipped_uframes = uframes - (ehci->maxframes * 8);
+            ehci_update_frindex(ehci, skipped_uframes);
+            ehci->last_run_ns += UFRAME_TIMER_NS * skipped_uframes;
+            uframes -= skipped_uframes;
+            DPRINTF("WARNING - EHCI skipped %d uframes\n", skipped_uframes);
         }
 
-        for (i = 0; i < frames; i++) {
+        for (i = 0; i < uframes; i++) {
             /*
              * If we're running behind schedule, we should not catch up
              * too fast, as that will make some guests unhappy:
-             * 1) We must process a minimum of MIN_FR_PER_TICK frames,
+             * 1) We must process a minimum of MIN_UFR_PER_TICK frames,
              *    otherwise we will never catch up
              * 2) Process frames until the guest has requested an irq (IOC)
              */
-            if (i >= MIN_FR_PER_TICK) {
+            if (i >= MIN_UFR_PER_TICK) {
                 ehci_commit_irq(ehci);
                 if ((ehci->usbsts & USBINTR_MASK) & ehci->usbintr) {
                     break;
@@ -2306,13 +2316,15 @@ static void ehci_frame_timer(void *opaque)
                 ehci->periodic_sched_active--;
             }
             ehci_update_frindex(ehci, 1);
-            ehci_advance_periodic_state(ehci);
-            ehci->last_run_ns += FRAME_TIMER_NS;
+            if ((ehci->frindex & 7) == 0) {
+                ehci_advance_periodic_state(ehci);
+            }
+            ehci->last_run_ns += UFRAME_TIMER_NS;
         }
     } else {
         ehci->periodic_sched_active = 0;
-        ehci_update_frindex(ehci, frames);
-        ehci->last_run_ns += FRAME_TIMER_NS * frames;
+        ehci_update_frindex(ehci, uframes);
+        ehci->last_run_ns += UFRAME_TIMER_NS * uframes;
     }
 
     if (ehci->periodic_sched_active) {
@@ -2391,6 +2403,17 @@ static USBBusOps ehci_bus_ops = {
     .wakeup_endpoint = ehci_wakeup_endpoint,
 };
 
+static void usb_ehci_pre_save(void *opaque)
+{
+    EHCIState *ehci = opaque;
+    uint32_t new_frindex;
+
+    /* Round down frindex to a multiple of 8 for migration compatibility */
+    new_frindex = ehci->frindex & ~7;
+    ehci->last_run_ns -= (ehci->frindex - new_frindex) * UFRAME_TIMER_NS;
+    ehci->frindex = new_frindex;
+}
+
 static int usb_ehci_post_load(void *opaque, int version_id)
 {
     EHCIState *s = opaque;
@@ -2441,6 +2464,7 @@ const VMStateDescription vmstate_ehci = {
     .name        = "ehci-core",
     .version_id  = 2,
     .minimum_version_id  = 1,
+    .pre_save    = usb_ehci_pre_save,
     .post_load   = usb_ehci_post_load,
     .fields      = (VMStateField[]) {
         /* mmio registers */
-- 
1.8.0.1