[Qemu-devel] [PATCH v2 1/2] ohci: delay first SOF interrupt

[Laurent Vivier <lvivier@redhat.com>]

On overcommitted CPU, kernel can be so slow that an interrupt can
be triggered by the device whereas the driver is not ready to receive
it. This drives us into an infinite loop.

This does not happen on real hardware because real hardware never send
interrupt immediately after the controller has been moved to OPERATION state.

This patch tries to delay the first SOF interrupt to let driver exits from
the critical section (which is not protected against interrupts...)

Some details:

- ohci_irq(): the OHCI interrupt handler, acknowledges the SOF IRQ
  only if the state of the driver (rh_state) is OHCI_STATE_RUNNING.
  So if this interrupt happens and the driver is not in this state,
  the function is called again and again, moving the system to a
  CPU starvation.

- ohci_rh_resume(): the driver re-enables operation with OHCI_USB_OPER.
  In QEMU this start the SOF timer and QEMU starts to send IRQs. As
  the driver is not in OHCI_STATE_RUNNING and not protected against IRQ,
  the ohci_irq() can be called and the driver never moved to
  OHCI_STATE_RUNNING.

Suggested-by: Gerd Hoffmann <kraxel@redhat.com>
Signed-off-by: Laurent Vivier <lvivier@redhat.com>
---
 hw/usb/hcd-ohci.c | 16 +++++++++++++---
 1 file changed, 13 insertions(+), 3 deletions(-)

diff --git a/hw/usb/hcd-ohci.c b/hw/usb/hcd-ohci.c
index 7d65818..a44fab2 100644
--- a/hw/usb/hcd-ohci.c
+++ b/hw/usb/hcd-ohci.c
@@ -1231,11 +1231,16 @@ static int ohci_service_ed_list(OHCIState *ohci, uint32_t head, int completion)
     return active;
 }
 
-/* Generate a SOF event, and set a timer for EOF */
-static void ohci_sof(OHCIState *ohci)
+/* set a timer for EOF */
+static void ohci_eof_timer(OHCIState *ohci)
 {
     ohci->sof_time = qemu_clock_get_ns(QEMU_CLOCK_VIRTUAL);
     timer_mod(ohci->eof_timer, ohci->sof_time + usb_frame_time);
+}
+/* Set a timer for EOF and generate a SOF event */
+static void ohci_sof(OHCIState *ohci)
+{
+    ohci_eof_timer(ohci);
     ohci_set_interrupt(ohci, OHCI_INTR_SF);
 }
 
@@ -1343,7 +1348,12 @@ static int ohci_bus_start(OHCIState *ohci)
 
     trace_usb_ohci_start(ohci->name);
 
-    ohci_sof(ohci);
+    /* Delay the first SOF event by one frame time as
+     * linux driver is not ready to receive it and
+     * can meet some race conditions
+     */
+
+    ohci_eof_timer(ohci);
 
     return 1;
 }
-- 
1.8.3.1