[PATCH V4 15/24] xen/arm: Stick around in leave_hypervisor_to_guest until I/O has completed

[Oleksandr Tyshchenko <olekstysh@xxxxxxxxx>]

From: Oleksandr Tyshchenko <oleksandr_tyshchenko@xxxxxxxx>

This patch adds proper handling of return value of
vcpu_ioreq_handle_completion() which involves using a loop in
leave_hypervisor_to_guest().

The reason to use an unbounded loop here is the fact that vCPU shouldn't
continue until the I/O has completed.

The IOREQ code is using wait_on_xen_event_channel(). Yet, this can
still "exit" early if an event has been received. But this doesn't mean
the I/O has completed (in can be just a spurious wake-up). So we need
to check if the I/O has completed and wait again if it hasn't (we will
block the vCPU again until an event is received). This loop makes sure
that all the vCPU works are done before we return to the guest.

The call chain below:
check_for_vcpu_work -> vcpu_ioreq_handle_completion -> wait_for_io ->
wait_on_xen_event_channel

The worse that can happen here if the vCPU will never run again
(the I/O will never complete). But, in Xen case, if the I/O never
completes then it most likely means that something went horribly
wrong with the Device Emulator. And it is most likely not safe
to continue. So letting the vCPU to spin forever if the I/O never
completes is a safer action than letting it continue and leaving
the guest in unclear state and is the best what we can do for now.

Please note, using this loop we will not spin forever on a pCPU,
preventing any other vCPUs from being scheduled. At every loop
we will call check_for_pcpu_work() that will process pending
softirqs. In case of failure, the guest will crash and the vCPU
will be unscheduled. In normal case, if the rescheduling is necessary
(might be set by a timer or by a caller in check_for_vcpu_work(),
where wait_for_io() is a preemption point) the vCPU will be rescheduled
to give place to someone else.

Signed-off-by: Oleksandr Tyshchenko <oleksandr_tyshchenko@xxxxxxxx>
CC: Julien Grall <julien.grall@xxxxxxx>
[On Arm only]
Tested-by: Wei Chen <Wei.Chen@xxxxxxx>

---
Please note, this is a split/cleanup/hardening of Julien's PoC:
"Add support for Guest IO forwarding to a device emulator"

Changes V1 -> V2:
   - new patch, changes were derived from (+ new explanation):
     arm/ioreq: Introduce arch specific bits for IOREQ/DM features

Changes V2 -> V3:
   - update patch description

Changes V3 -> V4:
   - update patch description and comment in code
---
 xen/arch/arm/traps.c | 38 +++++++++++++++++++++++++++++++++-----
 1 file changed, 33 insertions(+), 5 deletions(-)

diff --git a/xen/arch/arm/traps.c b/xen/arch/arm/traps.c
index 036b13f..4a83e1e 100644
--- a/xen/arch/arm/traps.c
+++ b/xen/arch/arm/traps.c
@@ -2257,18 +2257,23 @@ static void check_for_pcpu_work(void)
  * Process pending work for the vCPU. Any call should be fast or
  * implement preemption.
  */
-static void check_for_vcpu_work(void)
+static bool check_for_vcpu_work(void)
 {
     struct vcpu *v = current;
 
 #ifdef CONFIG_IOREQ_SERVER
+    bool handled;
+
     local_irq_enable();
-    vcpu_ioreq_handle_completion(v);
+    handled = vcpu_ioreq_handle_completion(v);
     local_irq_disable();
+
+    if ( !handled )
+        return true;
 #endif
 
     if ( likely(!v->arch.need_flush_to_ram) )
-        return;
+        return false;
 
     /*
      * Give a chance for the pCPU to process work before handling the vCPU
@@ -2279,6 +2284,8 @@ static void check_for_vcpu_work(void)
     local_irq_enable();
     p2m_flush_vm(v);
     local_irq_disable();
+
+    return false;
 }
 
 /*
@@ -2291,8 +2298,29 @@ void leave_hypervisor_to_guest(void)
 {
     local_irq_disable();
 
-    check_for_vcpu_work();
-    check_for_pcpu_work();
+    /*
+     * The reason to use an unbounded loop here is the fact that vCPU
+     * shouldn't continue until the I/O has completed.
+     *
+     * The worse that can happen here if the vCPU will never run again
+     * (the I/O will never complete). But, in Xen case, if the I/O never
+     * completes then it most likely means that something went horribly
+     * wrong with the Device Emulator. And it is most likely not safe
+     * to continue. So letting the vCPU to spin forever if the I/O never
+     * completes is a safer action than letting it continue and leaving
+     * the guest in unclear state and is the best what we can do for now.
+     *
+     * Please note, using this loop we will not spin forever on a pCPU,
+     * preventing any other vCPUs from being scheduled. At every loop
+     * we will call check_for_pcpu_work() that will process pending
+     * softirqs. In case of failure, the guest will crash and the vCPU
+     * will be unscheduled. In normal case, if the rescheduling is necessary
+     * (might be set by a timer or by a caller in check_for_vcpu_work(),
+     * the vCPU will be rescheduled to give place to someone else.
+     */
+    do {
+        check_for_pcpu_work();
+    } while ( check_for_vcpu_work() );
 
     vgic_sync_to_lrs();
 
-- 
2.7.4