RE: [PATCH 07/11] PCI: hv: Prepare hv_compose_msi_msg() for the VMBus-channel-interrupt-to-vCPU reassignment functionality

[Michael Kelley <mikelley@microsoft.com>]

From: Andrea Parri (Microsoft) <parri.andrea@gmail.com> 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 <mikelley@microsoft.com>
> Signed-off-by: Andrea Parri (Microsoft) <parri.andrea@gmail.com>
> Cc: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
> Cc: Andrew Murray <amurray@thegoodpenguin.co.uk>
> Cc: Bjorn Helgaas <bhelgaas@google.com>
> Cc: <linux-pci@vger.kernel.org>
> ---
>  drivers/pci/controller/pci-hyperv.c | 44 ++++++++++++++++++-----------
>  1 file changed, 28 insertions(+), 16 deletions(-)
> 

Reviewed-by: Michael Kelley <mikelley@microsoft.com>