Re: [PATCH v4 3/3] balloon: add VIRTIO_BALLOON_F_DEVICE_INIT_ON_INFLATE

["Michael S. Tsirkin" <mst@redhat.com>]

On Fri, May 15, 2026 at 12:09:24PM +0300, Manos Pitsidianakis wrote:
> On Thu, May 7, 2026 at 10:52 AM Michael S. Tsirkin <mst@redhat.com> wrote:
> >
> > Add VIRTIO_BALLOON_F_DEVICE_INIT_ON_INFLATE (bit 7): the device
> > initializes inflated pages and returns a per-page bitmap indicating
> > which pages were successfully initialized.
> >
> > Update Security Considerations Data Integrity subsection to cover
> > both DEVICE_INIT_REPORTED and DEVICE_INIT_ON_INFLATE.
> >
> > Fixes: https://github.com/oasis-tcs/virtio-spec/issues/242
> > Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
> > ---
> >  device-types/balloon/description.tex | 73 +++++++++++++++++++++++++++-
> >  1 file changed, 71 insertions(+), 2 deletions(-)
> >
> > diff --git a/device-types/balloon/description.tex b/device-types/balloon/description.tex
> > index fa33105..7afdb24 100644
> > --- a/device-types/balloon/description.tex
> > +++ b/device-types/balloon/description.tex
> > @@ -52,6 +52,9 @@ \subsection{Feature bits}\label{sec:Device Types / Memory Balloon Device / Featu
> >  \item[ VIRTIO_BALLOON_F_DEVICE_INIT_REPORTED(6) ] The device initializes
> >      reported pages.
> >
> > +\item[ VIRTIO_BALLOON_F_DEVICE_INIT_ON_INFLATE(7) ] The device initializes
> > +    inflated pages.
> > +
> >  \end{description}
> >
> >  \subsubsection{Feature bit requirements}\label{sec:Device Types / Memory Balloon Device / Feature bits / Feature bit requirements}
> > @@ -249,6 +252,10 @@ \subsection{Device Operation}\label{sec:Device Types / Memory Balloon Device / D
> >  and before detecting its physical number in a deflate request
> >  and acknowledging the deflate request.
> >
> > +If VIRTIO_BALLOON_F_DEVICE_INIT_ON_INFLATE is negotiated, the
> > +device MUST NOT modify a page after reporting it as initialized
> > +in the bitmap.
> > +
> >  \paragraph{Legacy Interface: Device Operation}\label{sec:Device
> >  Types / Memory Balloon Device / Device Operation / Legacy
> >  Interface: Device Operation}
> > @@ -672,6 +679,67 @@ \subsubsection{Device Initialized Reported Pages}\label{sec:Device Types / Memor
> >  If VIRTIO_BALLOON_F_PAGE_POISON is not negotiated, the device
> >  MUST fill each initialized page with zeros.
> >
> > +\subsubsection{Device Initialized Pages on Inflate}\label{sec:Device Types / Memory Balloon Device / Device Operation / Device Initialized Pages on Inflate}
> > +
> > +When VIRTIO_BALLOON_F_DEVICE_INIT_ON_INFLATE is negotiated, the
> > +device initializes inflated pages and reports which pages were
> > +successfully initialized via a per-page bitmap.
> > +
> > +The driver appends a device-writable bitmap buffer to each inflate
> > +descriptor chain, after the PFN outbuf.
> 
> Maybe we could say "page physical number" instead of PFN to stay
> consistent with the rest of the balloon spec.
> 
> > The bitmap contains one
> > +bit per balloon page (4KB), where bit N corresponds to the Nth PFN
> > +in the inflate request (0-indexed). Bit N is stored as bit (N mod 8)
> > +of byte (N / 8), with bit 0 being the least significant bit.
> > +The device sets a bit to 1 if it successfully initialized the
> > +corresponding page, or 0 if it did not.
> 
> Does this mean page size cannot be larger than 4kb?

With legacy balloon, pretty much.
This hack of sticking right-shifted PFNs in an outbuf
is a broken interface that can't even support large guests.
If someone worked on a better one, would be great.
But for now, this just supports zero reporting with that.

-- 
MST