On Wed, Mar 11, 2015 at 11:03:14PM +0100, Greg Kurz wrote:
> On Wed, 11 Mar 2015 21:06:05 +0100
> "Michael S. Tsirkin" <mst@xxxxxxxxxx> wrote:
>
> > On Wed, Mar 11, 2015 at 07:04:38PM +0100, Greg Kurz wrote:
> > > vhost is seriously broken with ppc64le guests, even in the supposedly
> > > supported case where the host is ppc64le and we don't need cross-endian
> > > support.
> > >
> > > The TX virtqueue fails to be handled by vhost and falls back to QEMU.
> > > Despite this unexpected scenario where RX is vhost and TX is QEMU, the
> > > guest runs well with reduced upload performances... until you reboot,
> > > migrate, managed save or in fact any operation that causes vhost_net
> > > to be re-started. Network connectivity is then permanantly lost for
> > > the guest.
> > >
> > > TX falling back to QEMU is the result of a failed MMIO store emulation
> > > in KVM. Debugging shows that:
> > >
> > > kvmppc_emulate_mmio()
> > > |
> > > +-> kvmppc_handle_store()
> > > |
> > > +-> kvm_io_bus_write()
> > > |
> > > +-> __kvm_io_bus_write() returns -EOPNOTSUPP
> > >
> > > This happens because no matching device was found:
> > >
> > > __kvm_io_bus_write()
> > > |
> > > +->kvm_iodevice_write()
> > > |
> > > +->ioeventfd_write()
> > > |
> > > +->ioeventfd_in_range() returns false for all registered vrings
> > >
> > > Extra debugging shows that the TX vring number (16-bit) is supposed to
> > > be 0x0100 but QEMU passes 0x0001 to KVM... This happens *again* because
> > > QEMU still assumes powerpc is big endian (TARGET_WORDS_BIGENDIAN) by
> > > default.
> > >
> > > This patch adds an extra swap in virtio_pci_set_host_notifier_internal()
> > > to negate the one that is done in adjust_endianness(). Since this is not
> > > a hot path and we want to keep virtio-pci.o in common-obj, we don't care
> > > whether the guest is bi-endian or not.
> > >
> > > Reported-by: Cédric Le Goater <clg@xxxxxxxxxx>
> > > Suggested-by: Michael Roth <mdroth@xxxxxxxxxxxxxxxxxx>
> > > Signed-off-by: Greg Kurz <gkurz@xxxxxxxxxxxxxxxxxx>
> >
> > I am confused.
> > The value that notifications use is always LE.
>
> True but adjust_endianness() does swap unconditionally for ppc64
> because of TARGET_WORDS_BIGENDIAN.
>
> > Can't we avoid multiple swaps?
>
> That would mean adding an extra endianness argument down to
> memory_region_wrong_endianness()... not sure we want to do that.
>
> > They make my head spin.
> >
>
> I understand that the current fixed target endianness paradigm
> is best suited for most architectures. Extra swaps in specific
> non-critical locations allows to support odd beasts like ppc64le
> and arm64be without trashing more common paths. Maybe I can add a
> comment for better clarity (see below).
But common header format is simple, it's always LE.
It does not depend on target.
To me this looks like a bug in memory_region_add_eventfd,
it should do the right thing depending on device
endian-ness.
> > > ---
> > >
> > > I guess it is also a fix for virtio-1 but I didn't check.
> > >
> > > hw/virtio/virtio-pci.c | 11 +++++++++--
> > > 1 file changed, 9 insertions(+), 2 deletions(-)
> > >
> > > diff --git a/hw/virtio/virtio-pci.c b/hw/virtio/virtio-pci.c
> > > index e7baf7b..62b04c9 100644
> > > --- a/hw/virtio/virtio-pci.c
> > > +++ b/hw/virtio/virtio-pci.c
> > > @@ -133,6 +133,11 @@ static int virtio_pci_load_queue(DeviceState *d, int n, QEMUFile *f)
> > > return 0;
> > > }
> > >
>
> /* The host notifier will be swapped in adjust_endianness() according to the
> * target default endianness. We need to negate this swap if the device uses
> * an endianness that is not the default (ppc64le for example).
> */
>
> > > +static uint16_t cpu_to_host_notifier16(VirtIODevice *vdev, uint16_t val)
> > > +{
> > > + return virtio_is_big_endian(vdev) ? val : bswap16(val);
> > > +}
> > > +
> > > static int virtio_pci_set_host_notifier_internal(VirtIOPCIProxy *proxy,
> > > int n, bool assign, bool set_handler)
> > > {
> > > @@ -150,10 +155,12 @@ static int virtio_pci_set_host_notifier_internal(VirtIOPCIProxy *proxy,
> > > }
> > > virtio_queue_set_host_notifier_fd_handler(vq, true, set_handler);
> > > memory_region_add_eventfd(&proxy->bar, VIRTIO_PCI_QUEUE_NOTIFY, 2,
> > > - true, n, notifier);
> > > + true, cpu_to_host_notifier16(vdev, n),
> > > + notifier);
> > > } else {
> > > memory_region_del_eventfd(&proxy->bar, VIRTIO_PCI_QUEUE_NOTIFY, 2,
> > > - true, n, notifier);
> > > + true, cpu_to_host_notifier16(vdev, n),
> > > + notifier);
> > > virtio_queue_set_host_notifier_fd_handler(vq, false, false);
> > > event_notifier_cleanup(notifier);
> > > }
> >
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