On Thu, Apr 07, 2011 at 10:43:25AM -0700, Dave Hansen wrote: > > The virtio balloon driver has a VIRTIO_BALLOON_F_MUST_TELL_HOST > feature bit. Whenever the bit is set, we must always tell the > host before we free pages back to the allocator. Without this > 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, we are under no obligation to > reverse the order. Furthermore, all modern qemus set this bit.
Which qemus do this, specifically? Amit Shah just pointed out to me that upstream qemu.git and qemu-kvm.git don't seem to do this. Which qemu did you test this with? > So, the "tell second" code is completely unused and untestable. > Quoting Anthony: "untested code is broken code". > > This _also_ means that we don't have to preserve a pfn list > after the pages are freed, which should let us get rid of some > temporary storage (vb->pfns) eventually. > > > Signed-off-by: Dave Hansen <[email protected]> > --- > > linux-2.6.git-dave/drivers/virtio/virtio_balloon.c | 21 > ++++++++------------- > 1 file changed, 8 insertions(+), 13 deletions(-) > > diff -puN drivers/virtio/virtio_balloon.c~kill-tell-host-first-logic > drivers/virtio/virtio_balloon.c > --- linux-2.6.git/drivers/virtio/virtio_balloon.c~kill-tell-host-first-logic > 2011-04-07 10:23:12.016343374 -0700 > +++ linux-2.6.git-dave/drivers/virtio/virtio_balloon.c 2011-04-07 > 10:23:12.024343370 -0700 > @@ -40,9 +40,6 @@ struct virtio_balloon > /* Waiting for host to ack the pages we released. */ > struct completion acked; > > - /* Do we have to tell Host *before* we reuse pages? */ > - bool tell_host_first; > - > /* The pages we've told the Host we're not using. */ > unsigned int num_pages; > struct list_head pages; > @@ -151,13 +148,14 @@ static void leak_balloon(struct virtio_b > vb->num_pages--; > } > > - if (vb->tell_host_first) { > - tell_host(vb, vb->deflate_vq); > - release_pages_by_pfn(vb->pfns, vb->num_pfns); > - } else { > - release_pages_by_pfn(vb->pfns, vb->num_pfns); > - tell_host(vb, vb->deflate_vq); > - } > + > + /* > + * Note that if > + * virtio_has_feature(vdev, VIRTIO_BALLOON_F_MUST_TELL_HOST); > + * is true, we *have* to do it in this order > + */ > + tell_host(vb, vb->deflate_vq); > + release_pages_by_pfn(vb->pfns, vb->num_pfns); > } > > static inline void update_stat(struct virtio_balloon *vb, int idx, > @@ -325,9 +323,6 @@ static int virtballoon_probe(struct virt > goto out_del_vqs; > } > > - vb->tell_host_first > - = virtio_has_feature(vdev, VIRTIO_BALLOON_F_MUST_TELL_HOST); > - > return 0; > > out_del_vqs: > _ _______________________________________________ Virtualization mailing list [email protected] https://lists.linux-foundation.org/mailman/listinfo/virtualization
