[PATCH V2 15/23] 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 handle_io_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 an I/O has completed. In Xen case, if an 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 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.

This wouldn't be an issue for Xen as do_softirq() would be called at
every loop. In case of failure, the guest will crash and the vCPU
will be unscheduled.

Signed-off-by: Oleksandr Tyshchenko <oleksandr_tyshchenko@xxxxxxxx>
CC: Julien Grall <julien.grall@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
---
 xen/arch/arm/traps.c | 31 ++++++++++++++++++++++++++-----
 1 file changed, 26 insertions(+), 5 deletions(-)

diff --git a/xen/arch/arm/traps.c b/xen/arch/arm/traps.c
index b154837..507c095 100644
--- a/xen/arch/arm/traps.c
+++ b/xen/arch/arm/traps.c
@@ -2256,18 +2256,23 @@ 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();
-    handle_io_completion(v);
+    handled = handle_io_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
@@ -2278,6 +2283,8 @@ static void check_for_vcpu_work(void)
     local_irq_enable();
     p2m_flush_vm(v);
     local_irq_disable();
+
+    return false;
 }
 
 /*
@@ -2290,8 +2297,22 @@ 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 an I/O has completed. In Xen case, if an 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 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.
+     *
+     * This wouldn't be an issue for Xen as do_softirq() would be called at
+     * every loop. In case of failure, the guest will crash and the vCPU
+     * will be unscheduled.
+     */
+    do {
+        check_for_pcpu_work();
+    } while ( check_for_vcpu_work() );
 
     vgic_sync_to_lrs();
 
-- 
2.7.4