On Tue, Nov 19, 2019 at 06:50:03PM -0600, Michael Roth wrote:
> Currently the SLOF firmware for pseries guests will disable/re-enable
> a PCI device multiple times via IO/MEM/MASTER bits of PCI_COMMAND
> register after the initial probe/feature negotiation, as it tends to
> work with a single device at a time at various stages like probing
> and running block/network bootloaders without doing a full reset
> in-between.
>
> In QEMU, when PCI_COMMAND_MASTER is disabled we disable the
> corresponding IOMMU memory region, so DMA accesses (including to vring
> fields like idx/flags) will no longer undergo the necessary
> translation. Normally we wouldn't expect this to happen since it would
> be misbehavior on the driver side to continue driving DMA requests.
>
> However, in the case of pseries, with iommu_platform=on, we trigger the
> following sequence when tearing down the virtio-blk dataplane ioeventfd
> in response to the guest unsetting PCI_COMMAND_MASTER:
>
> #2 0x0000555555922651 in virtqueue_map_desc
> (vdev=vdev@entry=0x555556dbcfb0, p_num_sg=p_num_sg@entry=0x7fffe657e1a8,
> addr=addr@entry=0x7fffe657e240, iov=iov@entry=0x7fffe6580240,
> max_num_sg=max_num_sg@entry=1024, is_write=is_write@entry=false, pa=0, sz=0)
> at /home/mdroth/w/qemu.git/hw/virtio/virtio.c:757
> #3 0x0000555555922a89 in virtqueue_pop (vq=vq@entry=0x555556dc8660,
> sz=sz@entry=184)
> at /home/mdroth/w/qemu.git/hw/virtio/virtio.c:950
> #4 0x00005555558d3eca in virtio_blk_get_request (vq=0x555556dc8660,
> s=0x555556dbcfb0)
> at /home/mdroth/w/qemu.git/hw/block/virtio-blk.c:255
> #5 0x00005555558d3eca in virtio_blk_handle_vq (s=0x555556dbcfb0,
> vq=0x555556dc8660)
> at /home/mdroth/w/qemu.git/hw/block/virtio-blk.c:776
> #6 0x000055555591dd66 in virtio_queue_notify_aio_vq
> (vq=vq@entry=0x555556dc8660)
> at /home/mdroth/w/qemu.git/hw/virtio/virtio.c:1550
> #7 0x000055555591ecef in virtio_queue_notify_aio_vq (vq=0x555556dc8660)
> at /home/mdroth/w/qemu.git/hw/virtio/virtio.c:1546
> #8 0x000055555591ecef in virtio_queue_host_notifier_aio_poll
> (opaque=0x555556dc86c8)
> at /home/mdroth/w/qemu.git/hw/virtio/virtio.c:2527
> #9 0x0000555555d02164 in run_poll_handlers_once
> (ctx=ctx@entry=0x55555688bfc0, timeout=timeout@entry=0x7fffe65844a8)
> at /home/mdroth/w/qemu.git/util/aio-posix.c:520
> #10 0x0000555555d02d1b in try_poll_mode (timeout=0x7fffe65844a8,
> ctx=0x55555688bfc0)
> at /home/mdroth/w/qemu.git/util/aio-posix.c:607
> #11 0x0000555555d02d1b in aio_poll (ctx=ctx@entry=0x55555688bfc0,
> blocking=blocking@entry=true)
> at /home/mdroth/w/qemu.git/util/aio-posix.c:639
> #12 0x0000555555d0004d in aio_wait_bh_oneshot (ctx=0x55555688bfc0,
> cb=cb@entry=0x5555558d5130 <virtio_blk_data_plane_stop_bh>,
> opaque=opaque@entry=0x555556de86f0)
> at /home/mdroth/w/qemu.git/util/aio-wait.c:71
> #13 0x00005555558d59bf in virtio_blk_data_plane_stop (vdev=<optimized out>)
> at /home/mdroth/w/qemu.git/hw/block/dataplane/virtio-blk.c:288
> #14 0x0000555555b906a1 in virtio_bus_stop_ioeventfd
> (bus=bus@entry=0x555556dbcf38)
> at /home/mdroth/w/qemu.git/hw/virtio/virtio-bus.c:245
> #15 0x0000555555b90dbb in virtio_bus_stop_ioeventfd
> (bus=bus@entry=0x555556dbcf38)
> at /home/mdroth/w/qemu.git/hw/virtio/virtio-bus.c:237
> #16 0x0000555555b92a8e in virtio_pci_stop_ioeventfd (proxy=0x555556db4e40)
> at /home/mdroth/w/qemu.git/hw/virtio/virtio-pci.c:292
> #17 0x0000555555b92a8e in virtio_write_config (pci_dev=0x555556db4e40,
> address=<optimized out>, val=1048832, len=<optimized out>)
> at /home/mdroth/w/qemu.git/hw/virtio/virtio-pci.c:613
>
> I.e. the calling code is only scheduling a one-shot BH for
> virtio_blk_data_plane_stop_bh, but somehow we end up trying to process
> an additional virtqueue entry before we get there. This is likely due
> to the following check in virtio_queue_host_notifier_aio_poll:
>
> static bool virtio_queue_host_notifier_aio_poll(void *opaque)
> {
> EventNotifier *n = opaque;
> VirtQueue *vq = container_of(n, VirtQueue, host_notifier);
> bool progress;
>
> if (!vq->vring.desc || virtio_queue_empty(vq)) {
> return false;
> }
>
> progress = virtio_queue_notify_aio_vq(vq);
>
> namely the call to virtio_queue_empty(). In this case, since no new
> requests have actually been issued, shadow_avail_idx == last_avail_idx,
> so we actually try to access the vring via vring_avail_idx() to get
> the latest non-shadowed idx:
>
> int virtio_queue_empty(VirtQueue *vq)
> {
> bool empty;
> ...
>
> if (vq->shadow_avail_idx != vq->last_avail_idx) {
> return 0;
> }
>
> rcu_read_lock();
> empty = vring_avail_idx(vq) == vq->last_avail_idx;
> rcu_read_unlock();
> return empty;
>
> but since the IOMMU region has been disabled we get a bogus value (0
> usually), which causes virtio_queue_empty() to falsely report that
> there are entries to be processed, which causes errors such as:
>
> "virtio: zero sized buffers are not allowed"
>
> or
>
> "virtio-blk missing headers"
>
> and puts the device in an error state.
>
> This patch works around the issue by introducing virtio_set_disabled(),
> which sets a 'disabled' flag to bypass checks like virtio_queue_empty()
> when bus-mastering is disabled. Since we'd check this flag at all the
> same sites as vdev->broken, we replace those checks with an inline
> function which checks for either vdev->broken or vdev->disabled.
>
> The 'disabled' flag is only migrated when set, which should be fairly
> rare, but to maintain migration compatibility we disable it's use for
> older machine types. Users requiring the use of the flag in conjunction
> with older machine types can set it explicitly as a virtio-device
> option.
>
> NOTES:
>
> - This leaves some other oddities in play, like the fact that
> DRIVER_OK also gets unset in response to bus-mastering being
> disabled, but not restored (however the device seems to continue
> working)
> - Similarly, we disable the host notifier via
> virtio_bus_stop_ioeventfd(), which seems to move the handling out
> of virtio-blk dataplane and back into the main IO thread, and it
> ends up staying there till a reset (but otherwise continues working
> normally)
>
> Cc: David Gibson <da...@gibson.dropbear.id.au>,
> Cc: Alexey Kardashevskiy <a...@ozlabs.ru>
> Cc: "Michael S. Tsirkin" <m...@redhat.com>
> Signed-off-by: Michael Roth <mdr...@linux.vnet.ibm.com>
> ---
> v2:
> - add migration support and only default to using 'disabled' flag
> for newer machines via virtio-device compat option (MST)
> - use inline functions to decouple checks from specific fields (MST)
> - rebased on master
So the only nit is: let's not make this part of
stable API, prefix the property with "x-".
Otherwise looks good.
> ---
> hw/core/machine.c | 1 +
> hw/virtio/virtio-pci.c | 12 ++++++++----
> hw/virtio/virtio.c | 35 ++++++++++++++++++++++++++++-------
> include/hw/virtio/virtio.h | 15 +++++++++++++++
> 4 files changed, 52 insertions(+), 11 deletions(-)
>
> diff --git a/hw/core/machine.c b/hw/core/machine.c
> index 1689ad3bf8..9f3073b23b 100644
> --- a/hw/core/machine.c
> +++ b/hw/core/machine.c
> @@ -29,6 +29,7 @@
>
> GlobalProperty hw_compat_4_1[] = {
> { "virtio-pci", "x-pcie-flr-init", "off" },
> + { "virtio-device", "use-disabled-flag", "false" },
> };
> const size_t hw_compat_4_1_len = G_N_ELEMENTS(hw_compat_4_1);
>
> diff --git a/hw/virtio/virtio-pci.c b/hw/virtio/virtio-pci.c
> index c6b47a9c73..394d409fb9 100644
> --- a/hw/virtio/virtio-pci.c
> +++ b/hw/virtio/virtio-pci.c
> @@ -608,10 +608,14 @@ static void virtio_write_config(PCIDevice *pci_dev,
> uint32_t address,
> pcie_cap_flr_write_config(pci_dev, address, val, len);
> }
>
> - if (range_covers_byte(address, len, PCI_COMMAND) &&
> - !(pci_dev->config[PCI_COMMAND] & PCI_COMMAND_MASTER)) {
> - virtio_pci_stop_ioeventfd(proxy);
> - virtio_set_status(vdev, vdev->status & ~VIRTIO_CONFIG_S_DRIVER_OK);
> + if (range_covers_byte(address, len, PCI_COMMAND)) {
> + if (!(pci_dev->config[PCI_COMMAND] & PCI_COMMAND_MASTER)) {
> + virtio_set_disabled(vdev, true);
> + virtio_pci_stop_ioeventfd(proxy);
> + virtio_set_status(vdev, vdev->status &
> ~VIRTIO_CONFIG_S_DRIVER_OK);
> + } else {
> + virtio_set_disabled(vdev, false);
> + }
> }
>
> if (proxy->config_cap &&
> diff --git a/hw/virtio/virtio.c b/hw/virtio/virtio.c
> index 04716b5f6c..3cb603a466 100644
> --- a/hw/virtio/virtio.c
> +++ b/hw/virtio/virtio.c
> @@ -546,7 +546,7 @@ static inline bool is_desc_avail(uint16_t flags, bool
> wrap_counter)
> * Called within rcu_read_lock(). */
> static int virtio_queue_empty_rcu(VirtQueue *vq)
> {
> - if (unlikely(vq->vdev->broken)) {
> + if (virtio_device_disabled(vq->vdev)) {
> return 1;
> }
>
> @@ -565,7 +565,7 @@ static int virtio_queue_split_empty(VirtQueue *vq)
> {
> bool empty;
>
> - if (unlikely(vq->vdev->broken)) {
> + if (virtio_device_disabled(vq->vdev)) {
> return 1;
> }
>
> @@ -783,7 +783,7 @@ void virtqueue_fill(VirtQueue *vq, const VirtQueueElement
> *elem,
>
> virtqueue_unmap_sg(vq, elem, len);
>
> - if (unlikely(vq->vdev->broken)) {
> + if (virtio_device_disabled(vq->vdev)) {
> return;
> }
>
> @@ -839,7 +839,7 @@ static void virtqueue_packed_flush(VirtQueue *vq,
> unsigned int count)
>
> void virtqueue_flush(VirtQueue *vq, unsigned int count)
> {
> - if (unlikely(vq->vdev->broken)) {
> + if (virtio_device_disabled(vq->vdev)) {
> vq->inuse -= count;
> return;
> }
> @@ -1602,7 +1602,7 @@ err_undo_map:
>
> void *virtqueue_pop(VirtQueue *vq, size_t sz)
> {
> - if (unlikely(vq->vdev->broken)) {
> + if (virtio_device_disabled(vq->vdev)) {
> return NULL;
> }
>
> @@ -1698,7 +1698,7 @@ unsigned int virtqueue_drop_all(VirtQueue *vq)
> {
> struct VirtIODevice *vdev = vq->vdev;
>
> - if (unlikely(vdev->broken)) {
> + if (virtio_device_disabled(vq->vdev)) {
> return 0;
> }
>
> @@ -1816,7 +1816,7 @@ static void virtio_notify_vector(VirtIODevice *vdev,
> uint16_t vector)
> BusState *qbus = qdev_get_parent_bus(DEVICE(vdev));
> VirtioBusClass *k = VIRTIO_BUS_GET_CLASS(qbus);
>
> - if (unlikely(vdev->broken)) {
> + if (virtio_device_disabled(vdev)) {
> return;
> }
>
> @@ -1920,6 +1920,7 @@ void virtio_reset(void *opaque)
> vdev->guest_features = 0;
> vdev->queue_sel = 0;
> vdev->status = 0;
> + vdev->disabled = false;
> atomic_set(&vdev->isr, 0);
> vdev->config_vector = VIRTIO_NO_VECTOR;
> virtio_notify_vector(vdev, vdev->config_vector);
> @@ -2553,6 +2554,13 @@ static bool virtio_started_needed(void *opaque)
> return vdev->started;
> }
>
> +static bool virtio_disabled_needed(void *opaque)
> +{
> + VirtIODevice *vdev = opaque;
> +
> + return vdev->disabled;
> +}
> +
> static const VMStateDescription vmstate_virtqueue = {
> .name = "virtqueue_state",
> .version_id = 1,
> @@ -2718,6 +2726,17 @@ static const VMStateDescription vmstate_virtio_started
> = {
> }
> };
>
> +static const VMStateDescription vmstate_virtio_disabled = {
> + .name = "virtio/disabled",
> + .version_id = 1,
> + .minimum_version_id = 1,
> + .needed = &virtio_disabled_needed,
> + .fields = (VMStateField[]) {
> + VMSTATE_BOOL(disabled, VirtIODevice),
> + VMSTATE_END_OF_LIST()
> + }
> +};
> +
> static const VMStateDescription vmstate_virtio = {
> .name = "virtio",
> .version_id = 1,
> @@ -2735,6 +2754,7 @@ static const VMStateDescription vmstate_virtio = {
> &vmstate_virtio_extra_state,
> &vmstate_virtio_started,
> &vmstate_virtio_packed_virtqueues,
> + &vmstate_virtio_disabled,
> NULL
> }
> };
> @@ -3569,6 +3589,7 @@ static void virtio_device_instance_finalize(Object *obj)
> static Property virtio_properties[] = {
> DEFINE_VIRTIO_COMMON_FEATURES(VirtIODevice, host_features),
> DEFINE_PROP_BOOL("use-started", VirtIODevice, use_started, true),
> + DEFINE_PROP_BOOL("use-disabled-flag", VirtIODevice, use_disabled_flag,
> true),
> DEFINE_PROP_END_OF_LIST(),
> };
>
> diff --git a/include/hw/virtio/virtio.h b/include/hw/virtio/virtio.h
> index c32a815303..f23d2efbc0 100644
> --- a/include/hw/virtio/virtio.h
> +++ b/include/hw/virtio/virtio.h
> @@ -100,6 +100,8 @@ struct VirtIODevice
> uint16_t device_id;
> bool vm_running;
> bool broken; /* device in invalid state, needs reset */
> + bool use_disabled_flag; /* allow use of 'disable' flag when needed */
> + bool disabled; /* device in temporarily disabled state */
> bool use_started;
> bool started;
> bool start_on_kick; /* when virtio 1.0 feature has not been negotiated */
> @@ -378,4 +380,17 @@ static inline void virtio_set_started(VirtIODevice
> *vdev, bool started)
> vdev->started = started;
> }
> }
> +
> +static inline void virtio_set_disabled(VirtIODevice *vdev, bool disable)
> +{
> + if (vdev->use_disabled_flag) {
> + vdev->disabled = disable;
> + }
> +}
> +
> +static inline bool virtio_device_disabled(VirtIODevice *vdev)
> +{
> + return unlikely(vdev->disabled || vdev->broken);
> +}
> +
> #endif
> --
> 2.17.1
