On Tue, Oct 27, 2026 at 02:26:33AM -0700, Ruben Hakobyan wrote: > On Mon, Oct 26, 2026 at 11:53 PM Michael S. Tsirkin <[email protected]> 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 <[email protected]> > > Cc: Ruben Hakobyan <[email protected]> > > Fixes: b0c504f15471 ("virtio-balloon: add support for providing free page > > reports to host") > > Cc: Alexander Duyck <[email protected]> > > Signed-off-by: Michael S. Tsirkin <[email protected]> > > --- > > > > 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.
Surprise removal of the balloon? Nope, it's a pre-existing issue that it's broken. Besides it's not really practical, no one removes the balloon - no need. > 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. Thanks a lot! Can you pls confirm whether the performance is about the same/better/worse? -- MST
