Quoting Michael S. Tsirkin (2019-11-13 04:09:02)
> On Tue, Nov 12, 2019 at 11:43:01PM -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 piggy-backs off the vdev->broken flag we already use to bypass
> > checks like virtio_queue_empty(), and sets/unsets it in response to
> > enabling/disabling bus-mastering.
> >
> > NOTES:
> >
> > - It's possible we could also work around this in SLOF by doing a
> > full reset instead of relying on PCI_COMMAND to enable/disable, but
> > in any case the QEMU behavior seems wrong.
> > - 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>
> > ---
> > hw/virtio/virtio-pci.c | 12 ++++++++----
> > hw/virtio/virtio.c | 7 ++++++-
> > include/hw/virtio/virtio.h | 1 +
> > 3 files changed, 15 insertions(+), 5 deletions(-)
> >
> > 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 527df03bfd..46580c357d 100644
> > --- a/hw/virtio/virtio.c
> > +++ b/hw/virtio/virtio.c
> > @@ -2575,6 +2575,11 @@ void virtio_device_set_child_bus_name(VirtIODevice
> > *vdev, char *bus_name)
> > vdev->bus_name = g_strdup(bus_name);
> > }
> >
> > +void virtio_set_disabled(VirtIODevice *vdev, bool disable)
> > +{
> > + vdev->broken = disable;
> > +}
> > +
> > void GCC_FMT_ATTR(2, 3) virtio_error(VirtIODevice *vdev, const char *fmt,
> > ...)
> > {
> > va_list ap;
>
> Hmm. I think just clear and immediate set of bus master while device was
> not doing any DMA actually should be fine and should not require a
> reset. Now it's true that right now guests reset first thing which will
> clear the broken flag, but I'd say it's wrong to require this specific
> order.
> I think the easiest thing is to add a "disabled" flag.
Agreed. Some comments on that below.
>
>
> > @@ -2588,7 +2593,7 @@ void GCC_FMT_ATTR(2, 3) virtio_error(VirtIODevice
> > *vdev, const char *fmt, ...)
> > virtio_notify_config(vdev);
> > }
> >
> > - vdev->broken = true;
> > + virtio_set_disabled(vdev, true);
> > }
> >
> > static void virtio_memory_listener_commit(MemoryListener *listener)
> > diff --git a/include/hw/virtio/virtio.h b/include/hw/virtio/virtio.h
> > index 48e8d04ff6..921945b7e8 100644
> > --- a/include/hw/virtio/virtio.h
> > +++ b/include/hw/virtio/virtio.h
> > @@ -168,6 +168,7 @@ void virtio_init(VirtIODevice *vdev, const char *name,
> > uint16_t device_id, size_t config_size);
> > void virtio_cleanup(VirtIODevice *vdev);
> >
> > +void virtio_set_disabled(VirtIODevice *vdev, bool disable);
> > void virtio_error(VirtIODevice *vdev, const char *fmt, ...)
> > GCC_FMT_ATTR(2, 3);
> >
> > /* Set the child bus name. */
>
>
> One open question here is cross version migration.
> What exactly happens if we migrate to an old qemu?
> from an old qemu?
Currently vdev->broken is already migrated via a subsection if it is set.
So, with the current proposed patch I would expect the following:
old -> new:
dev->broken set due to proposed virtio_set_disabled():
- N/A
dev->broken set due to other/existing reasons:
- value is migrated, migration succeeds
- device continues not working until a reset, or bus-mastering
enabled/re-enabled. (which is good, but also makes me realize
that a device that was put into broken state for reasons
other than virtio_set_disabled() should not get 'unbroken'
simply because bus-master bit was cycled. a separate flag
is probably needed)
- PASS
dev->broken not set:
- not migrated
- PASS
new -> old:
dev->broken set due to proposed virtio_set_disabled():
- migration succeeds to any QEMU that already has handling for
dev->broken.
- migration fails for any QEMU that doesn't have handling for
dev->broken as it does now, but will a higher chance of
triggering
- device won't work until it is reset. for most guests we will
probably get a reset before the device is used again anyway. for
the SLOF case the device will stay broken after bus-mastering
re-enabled, but that's the case for existing QEMU now anyway
- PASS, but with increased chance of migration breakage for
QEMU's that don't have handling for dev->broken.
dev->broken set due to other/existing reasons:
- migration succeeds to any QEMU that already has handling for
dev->broken. device will require a reset as it does now.
- migration fails for any QEMU that doesn't have handling for
dev->broken as it does now
- PASS
dev->broken not set:
- not migrated
- PASS
With a new dev->disabled flag, which we'd likely send using a subsection
like with dev->broken, I would expect the following:
old -> new:
dev->disabled set:
- N/A
dev->disabled not set:
- not migrated
- if source disables BM just before migration we will likely go
into an error state that either sets dev->broken or puts the
device in some other possibly bad state. either case would require
subsequent reset, just as they would without migration
- PASS
new -> old:
dev->disabled set:
- migration stream fails
- FAIL, but as expected, and doesn't seem common currently outside
of SLOF early-boot else we'd probably have more reports of
breakage from vring access while BM isn't set.
dev->broken not set:
- not migrated
- PASS
So re-using 'broken' is slightly better from a migration standpoint, but
as noted above it is probably wrong to unset 'broken' just because BM bit
gets cycled, so a new 'disabled' flag is probably needed.
We could probably get by with just adding a check for dev->disabled in
virtio_queue_empty(), or even earlier in
virtio_queue_host_notifier_aio_poll(), but it seems more proper to also
add it in most of the same places we currently check for dev->broken.
That seems somewhat redundant though, so I think maybe the best approach
is to:
- replace most dev->broken checks with checks for dev->disabled
- set dev->disabled whenever dev->broken gets set
- add a check in virtio_set_disabled() that only allows us to re-enable if
dev->broken hasn't also been set.
I'll work on a follow-up using that approach if it seems reasonable to
you.
Thanks!
>
> > --
> > 2.17.1
>
