Re: [PATCH v2 2/2] virtio_balloon: submit reported pages as individual buffers

[Ruben Hakobyan <hruben@meta.com>]

On Mon, Oct 26, 2026 at 11:53 PM Michael S. Tsirkin <mst@redhat.com> wrote:
> The virtio_balloon driver reports a scatter-gather list of free pages
> to the VMM so it may discard unused pages.
>
> For an arm64 kernel with 64 KiB pages, the default reporting and
> allocation orders allow entries ranging from 2 MiB to 512 MiB. If the
> report contains a sufficient number of large entries, we will end up
> with a descriptor chain that represents over 4 GiB of memory. This
> violates virtio spec, which says:
>         Drivers MUST NOT add a descriptor chain longer than $2^{32}$ bytes in total;
>
> Specifically, submitting $2^{32}$ or more breaks Rust-VMM.
>
> Submit each reported page as a separate virtqueue buffer instead of one
> buffer with an sg list of all pages.  This also avoids indirect
> descriptor allocation (kmalloc in the reporting path).
>
> To avoid multiple buffers causing interrupt storms, keep callbacks
> disabled while we wait for an event.
>
> TODO: as an alternative, keep using virtqueue_enable_cb_delayed
> but add buffers after there's space in the ring.
>
> Note: if the virtqueue is broken, wait_event on
> virtqueue_get_buf hangs.  This is a pre-existing issue:
> the old single-buffer code had the same hang.
>
> Reported-by: Ruben Hakobyan <hruben@meta.com>
> Cc: Ruben Hakobyan <hruben@meta.com>
> Fixes: b0c504f15471 ("virtio-balloon: add support for providing free page reports to host")
> Cc: Alexander Duyck <alexander.h.duyck@linux.intel.com>
> Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
> ---
>
> Notes (changelog):
>     changes since v1:
>         keep callbacks disabled while draining: get_buf re-arms the used
>         event for the next buffer, reintroducing an interrupt per page.
>         Disable the reporting vq callbacks at probe, and only arm the
>         delayed callback to wait.
>

Thanks for the patches! I think sashiko-bot found some issues on broken
queues in the first patch that are worth looking into for correctness.

Other than that, I tested the "normal path" on QEMU and Cloud Hypervisor,
both on 4 KiB and 64 KiB guests, and this works correctly.