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

On Fri, May 15, 2026 at 12:23=E2=80=AFPM Michael S. Tsirkin <[email protected]=
> wrote:
>
> On Fri, May 15, 2026 at 12:09:24PM +0300, Manos Pitsidianakis wrote:
> > On Thu, May 7, 2026 at 10:52=E2=80=AFAM 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 <[email protected]>
> > > ---
> > >  device-types/balloon/description.tex | 73 ++++++++++++++++++++++++++=
+-
> > >  1 file changed, 71 insertions(+), 2 deletions(-)
> > >
> > > diff --git a/device-types/balloon/description.tex b/device-types/ball=
oon/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 initial=
izes
> > >      reported pages.
> > >
> > > +\item[ VIRTIO_BALLOON_F_DEVICE_INIT_ON_INFLATE(7) ] The device initi=
alizes
> > > +    inflated pages.
> > > +
> > >  \end{description}
> > >
> > >  \subsubsection{Feature bit requirements}\label{sec:Device Types / Me=
mory Balloon Device / Feature bits / Feature bit requirements}
> > > @@ -249,6 +252,10 @@ \subsection{Device Operation}\label{sec:Device T=
ypes / 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 Page=
s 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.

Makes sense. Should we mention it in the traditional balloon device
description? Or even put device and driver MUST statements that page
size is always 4kb.

For this patch:

Reviewed-by: Manos Pitsidianakis <[email protected]>

>
> --
> MST
>