On Tue, Oct 27, 2026 at 2:35 AM Michael S. Tsirkin <[email protected]> wrote:
>
> On Tue, Oct 27, 2026 at 02:56:13AM +0000, Kevin Loughlin wrote:
> > In confidential computing guests requiring page state transitions,
> > unaccepted memory tracking operates at the EFI unaccepted memory unit
> > granularity (typically 2MB / order 9 on x86). Inflating the balloon
> > with 4KB pages would prevent unaccepting the backing 2MB unit.
> >
> > Inflate balloon pages at unaccepted_memory_order() when
> > virtballoon_cc_needs_state_transition() is true, while retaining
> > order-0 allocations for non-CoCo guests and CoCo guests that do not
> > use unaccepted memory transitions. When a higher-order balloon page
> > spans more 4KB balloon PFNs than VIRTIO_BALLOON_ARRAY_PFNS_MAX (256),
> > transmit its PFNs to the host in VIRTIO_BALLOON_ARRAY_PFNS_MAX chunks.
> >
> > Signed-off-by: Kevin Loughlin <[email protected]>
>
> I'm not too happy that this rides on the legacy PFN thing
> which is after all limited to 44 bit addresses.
>
> And given it is passing around 2MB chunks, then one 16 byte descriptor
> covering all of 2MB would actually be more efficient than 512 4-byte
> PFNs.
Agreed. I debated initially doing this in a single descriptor but was
trying to avoid also needing VMM changes.
Regardless, after reading yours and David's comments, I honestly think
it's better to just drop the legacy inflate/deflate and only include
the free page reporting functionality if we end up adding some form of
CoCo ballooning support. Beyond being inefficient-at-best here and
potentially making fragmentation worse as you noted in another
comment, the legacy inflate/deflate path is the biggest source of
bloat in the mm/ side of changes for this series.
> > ---
> > drivers/virtio/virtio_balloon.c | 87 ++++++++++++++++++++++++++-------
> > include/linux/balloon.h | 1 +
> > mm/balloon.c | 67 +++++++++++++++++++------
> > 3 files changed, 121 insertions(+), 34 deletions(-)
> >
> > diff --git a/drivers/virtio/virtio_balloon.c
> > b/drivers/virtio/virtio_balloon.c
> > index 11f74848fdc4..d162c162b496 100644
> > --- a/drivers/virtio/virtio_balloon.c
> > +++ b/drivers/virtio/virtio_balloon.c
> > @@ -182,6 +182,13 @@ static void balloon_ack(struct virtqueue *vq)
> > wake_up(&vb->acked);
> > }
> >
> > +static inline bool virtballoon_cc_needs_state_transition(void)
> > +{
> > + return IS_ENABLED(CONFIG_UNACCEPTED_MEMORY) &&
> > + cc_platform_has(CC_ATTR_GUEST_MEM_ENCRYPT) &&
> > + has_unaccepted_memory();
> > +}
> > +
> > static void tell_host(struct virtio_balloon *vb, struct virtqueue *vq)
> > {
> > struct scatterlist sg;
>
>
> Put all this, and preferred order, etc etc in kernel core somewhere,
> and expose some sensible APIs that devices can use.
Good call, will do.
