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    Subject[PATCH v2] virtio_balloon: Fix endless deflation and inflation on arm64
    Date
    The deflation request to the target, which isn't unaligned to the
    guest page size causes endless deflation and inflation actions. For
    example, we receive the flooding QMP events for the changes on memory
    balloon's size after a deflation request to the unaligned target is
    sent for the ARM64 guest, where we have 64KB base page size.

    /home/gavin/sandbox/qemu.main/build/qemu-system-aarch64 \
    -accel kvm -machine virt,gic-version=host -cpu host \
    -smp maxcpus=8,cpus=8,sockets=2,clusters=2,cores=2,threads=1 \
    -m 1024M,slots=16,maxmem=64G \
    -object memory-backend-ram,id=mem0,size=512M \
    -object memory-backend-ram,id=mem1,size=512M \
    -numa node,nodeid=0,memdev=mem0,cpus=0-3 \
    -numa node,nodeid=1,memdev=mem1,cpus=4-7 \
    : \
    -device virtio-balloon-pci,id=balloon0,bus=pcie.10

    { "execute" : "balloon", "arguments": { "value" : 1073672192 } }
    {"return": {}}
    {"timestamp": {"seconds": 1693272173, "microseconds": 88667}, \
    "event": "BALLOON_CHANGE", "data": {"actual": 1073610752}}
    {"timestamp": {"seconds": 1693272174, "microseconds": 89704}, \
    "event": "BALLOON_CHANGE", "data": {"actual": 1073610752}}
    {"timestamp": {"seconds": 1693272175, "microseconds": 90819}, \
    "event": "BALLOON_CHANGE", "data": {"actual": 1073610752}}
    {"timestamp": {"seconds": 1693272176, "microseconds": 91961}, \
    "event": "BALLOON_CHANGE", "data": {"actual": 1073610752}}
    {"timestamp": {"seconds": 1693272177, "microseconds": 93040}, \
    "event": "BALLOON_CHANGE", "data": {"actual": 1073676288}}
    {"timestamp": {"seconds": 1693272178, "microseconds": 94117}, \
    "event": "BALLOON_CHANGE", "data": {"actual": 1073676288}}
    {"timestamp": {"seconds": 1693272179, "microseconds": 95337}, \
    "event": "BALLOON_CHANGE", "data": {"actual": 1073610752}}
    {"timestamp": {"seconds": 1693272180, "microseconds": 96615}, \
    "event": "BALLOON_CHANGE", "data": {"actual": 1073676288}}
    {"timestamp": {"seconds": 1693272181, "microseconds": 97626}, \
    "event": "BALLOON_CHANGE", "data": {"actual": 1073610752}}
    {"timestamp": {"seconds": 1693272182, "microseconds": 98693}, \
    "event": "BALLOON_CHANGE", "data": {"actual": 1073676288}}
    {"timestamp": {"seconds": 1693272183, "microseconds": 99698}, \
    "event": "BALLOON_CHANGE", "data": {"actual": 1073610752}}
    {"timestamp": {"seconds": 1693272184, "microseconds": 100727}, \
    "event": "BALLOON_CHANGE", "data": {"actual": 1073610752}}
    {"timestamp": {"seconds": 1693272185, "microseconds": 90430}, \
    "event": "BALLOON_CHANGE", "data": {"actual": 1073610752}}
    {"timestamp": {"seconds": 1693272186, "microseconds": 102999}, \
    "event": "BALLOON_CHANGE", "data": {"actual": 1073676288}}
    :
    <The similar QMP events repeat>

    Fix it by aligning the target up to the guest page size, 64KB in this
    specific case. With this applied, no flooding QMP events are observed
    and the memory balloon's size can be stablizied to 0x3ffe0000 soon
    after the deflation request is sent.

    { "execute" : "balloon", "arguments": { "value" : 1073672192 } }
    {"return": {}}
    {"timestamp": {"seconds": 1693273328, "microseconds": 793075}, \
    "event": "BALLOON_CHANGE", "data": {"actual": 1073610752}}
    { "execute" : "query-balloon" }
    {"return": {"actual": 1073610752}}

    Signed-off-by: Gavin Shan <gshan@redhat.com>
    Tested-by: Zhenyu Zhang <zhenyzha@redhat.com>
    ---
    v2: Align @num_pages up to the guest page size in towards_target()
    directly as David suggested.
    ---
    drivers/virtio/virtio_balloon.c | 6 +++++-
    1 file changed, 5 insertions(+), 1 deletion(-)

    diff --git a/drivers/virtio/virtio_balloon.c b/drivers/virtio/virtio_balloon.c
    index 5b15936a5214..2d5d252ef419 100644
    --- a/drivers/virtio/virtio_balloon.c
    +++ b/drivers/virtio/virtio_balloon.c
    @@ -395,7 +395,11 @@ static inline s64 towards_target(struct virtio_balloon *vb)
    virtio_cread_le(vb->vdev, struct virtio_balloon_config, num_pages,
    &num_pages);

    - target = num_pages;
    + /*
    + * Aligned up to guest page size to avoid inflating and deflating
    + * balloon endlessly.
    + */
    + target = ALIGN(num_pages, VIRTIO_BALLOON_PAGES_PER_PAGE);
    return target - vb->num_pages;
    }

    --
    2.41.0
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