Add downstream patch for wait ack in free page hinting

[kalyazin]

In order to avoid potential corruption of guest pages due to a race condition under memory pressure in the guest, apply patch when building the kernel.

[cursor]

PR Summary

High Risk
Touches low-level virtio-balloon hinting synchronization and adds a new negotiated feature bit, which can introduce hangs/perf regressions if the device doesn’t ACK as expected or the feature is mis-negotiated.

Overview
Adds a downstream kernel patch that introduces VIRTIO_BALLOON_F_HINT_WAIT_ON_ACK to make free page hinting wait for a device ACK before moving hinted pages onto the guest’s free-page list.

When the feature is negotiated, the free-page virtqueue uses balloon_ack and the driver blocks after kicking (including start/stop command IDs) until the device returns a buffer; the feature is also validated to depend on VIRTIO_BALLOON_F_FREE_PAGE_HINT and is advertised in the driver feature list.

^{Reviewed by Cursor Bugbot for commit 5f96bc5. Bugbot is set up for automated code reviews on this repo. Configure here.}

[kalyazin]

PR Summary

Medium Risk Applies an out-of-tree kernel patch that changes virtio-balloon free page hinting behavior and can affect guest memory management/performance if the new feature is negotiated. Build flow now force-resets/cleans the kernel tree per version, which can fail or mask issues if tag resolution is wrong.

Overview The build now force-resets the linux source tree to the resolved tag and applies any patches/<version>/*.patch before running make olddefconfig, which can break builds if get_tag returns empty or if patches don’t apply cleanly.

A new downstream patch for 6.1.158 adds VIRTIO_BALLOON_F_HINT_WAIT_ON_ACK and makes free page hinting wait for a device ACK, which can increase hinting latency and potentially block if the device never acks.

Reviewed by Cursor Bugbot for commit 1baa63b. Bugbot is set up for automated code reviews on this repo. Configure here.

fixed in 7aba4fc