[Qemu-devel] [PATCH] set VIRTIO_BALLOON_F_MUST_TELL_HOST unconditionally

[Dave Hansen <dave@linux.vnet.ibm.com>]

There's a patch pending on LKML at the moment:

	http://lkml.org/lkml/2011/4/7/101

The virtio balloon driver has a VIRTIO_BALLOON_F_MUST_TELL_HOST
feature bit.  As of now, qemu-kvm defines the bit, but doesn't set it.
feature bit.  Whenever the bit is set, the guest kernel must always
tell the host before we free pages back to the allocator.  Without
this feature, we might free a page (and have another user touch it)
while the hypervisor is unprepared for it.

But, if the bit is _not_ set, there is no obligation to reverse the
order; we're under no obligation to do _anything_.

This patch makes the "tell host first" logic the only case.  This
should make everybody happy, and reduce the amount of untested or
untestable code in the kernel.

Signed-off-by: Dave Hansen <dave@linux.vnet.ibm.com>
Cc: Amit Shah <amit.shah@redhat.com>
Cc: Anthony Liguori <aliguori@linux.vnet.ibm.com>
---
 hw/virtio-balloon.c |    4 ++++
 1 files changed, 4 insertions(+), 0 deletions(-)

diff --git a/hw/virtio-balloon.c b/hw/virtio-balloon.c
index 1e6be18..71b0864 100644
--- a/hw/virtio-balloon.c
+++ b/hw/virtio-balloon.c
@@ -198,6 +198,10 @@ static void virtio_balloon_set_config(VirtIODevice *vdev,
 static uint32_t virtio_balloon_get_features(VirtIODevice *vdev, uint32_t f)
 {
     f |= (1 << VIRTIO_BALLOON_F_STATS_VQ);
+    /* Kernels >= 2.6.40 do this unconditionally, so set
+     * the bit to make old kernels do the same thing
+     */
+    f |= (1 << VIRTIO_BALLOON_F_MUST_TELL_HOST);
     return f;
 }
 
-- 
1.6.6.rc0.50.gaf06e9