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SubjectRE: [PATCH 07/11] PCI: hv: Prepare hv_compose_msi_msg() for the VMBus-channel-interrupt-to-vCPU reassignment functionality
From: Andrea Parri (Microsoft) <> Sent: Sunday, April 5, 2020 5:15 PM
> The current implementation of hv_compose_msi_msg() is incompatible with
> the new functionality that allows changing the vCPU a VMBus channel will
> interrupt: if this function always calls hv_pci_onchannelcallback() in
> the polling loop, the interrupt going to a different CPU could cause
> hv_pci_onchannelcallback() to be running simultaneously in a tasklet,
> which will break. The current code also has a problem in that it is not
> synchronized with vmbus_reset_channel_cb(): hv_compose_msi_msg() could
> be accessing the ring buffer via the call of hv_pci_onchannelcallback()
> well after the time that vmbus_reset_channel_cb() has finished.
> Fix these issues as follows. Disable the channel tasklet before
> entering the polling loop in hv_compose_msi_msg() and re-enable it when
> done. This will prevent hv_pci_onchannelcallback() from running in a
> tasklet on a different CPU. Moreover, poll by always calling
> hv_pci_onchannelcallback(), but check the channel callback function for
> NULL and invoke the callback within a sched_lock critical section. This
> will prevent hv_compose_msi_msg() from accessing the ring buffer after
> vmbus_reset_channel_cb() has acquired the sched_lock spinlock.
> Suggested-by: Michael Kelley <>
> Signed-off-by: Andrea Parri (Microsoft) <>
> Cc: Lorenzo Pieralisi <>
> Cc: Andrew Murray <>
> Cc: Bjorn Helgaas <>
> Cc: <>
> ---
> drivers/pci/controller/pci-hyperv.c | 44 ++++++++++++++++++-----------
> 1 file changed, 28 insertions(+), 16 deletions(-)

Reviewed-by: Michael Kelley <>

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