random: add interrupt callback to VMBus IRQ handler

commit 4b44f2d1 upstream.

The Hyper-V Linux Integration Services use the VMBus implementation for
communication with the Hypervisor. VMBus registers its own interrupt
handler that completely bypasses the common Linux interrupt handling.
This implies that the interrupt entropy collector is not triggered.

This patch adds the interrupt entropy collection callback into the VMBus
interrupt handler function.
Signed-off-by: Stephan Mueller <stephan.mueller@atsec.com>
Signed-off-by: Stephan Mueller <smueller@chronox.de>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
[bwh: Backported to 3.16: adjust context]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>

drivers/char/random.c

@@ -956,6 +956,7 @@ static void _xfer_secondary_pool(struct entropy_store *r, size_t nbytes)
 	mix_pool_bytes(r, tmp, bytes, NULL);
 	credit_entropy_bits(r, bytes*8);
 }
+EXPORT_SYMBOL_GPL(add_interrupt_randomness);
 
 /*
  * Used as a workqueue function so that when the input pool is getting

drivers/hv/vmbus_drv.c

@@ -33,6 +33,7 @@
 #include <linux/hyperv.h>
 #include <linux/kernel_stat.h>
 #include <linux/cpu.h>
+#include <linux/random.h>
 #include <asm/hyperv.h>
 #include <asm/hypervisor.h>
 #include <asm/mshyperv.h>
@@ -795,6 +796,8 @@ int __vmbus_driver_register(struct hv_driver *hv_driver, struct module *owner, c
 EXPORT_SYMBOL_GPL(__vmbus_driver_register);
 
 /**
+
+	add_interrupt_randomness(HYPERVISOR_CALLBACK_VECTOR, 0);
  * vmbus_driver_unregister() - Unregister a vmbus's driver
  * @drv: Pointer to driver structure you want to un-register
  *