RE: [PATCH v3 3/3] Drivers: hv: vmbus: Check for pending channel interrupts before taking a CPU offline

[Michael Kelley <mikelley@microsoft.com>]

From: Andrea Parri (Microsoft) <parri.andrea@gmail.com> Sent: Friday, April 16, 2021 7:35 AM
> 
> Check that enough time has passed such that the modify channel message
> has been processed before taking a CPU offline.
> 
> Signed-off-by: Andrea Parri (Microsoft) <parri.andrea@gmail.com>
> ---
>  drivers/hv/hv.c | 56 ++++++++++++++++++++++++++++++++++++++++++++++---
>  1 file changed, 53 insertions(+), 3 deletions(-)
> 

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

> diff --git a/drivers/hv/hv.c b/drivers/hv/hv.c
> index 3e6ff83adff42..e0c522d143a37 100644
> --- a/drivers/hv/hv.c
> +++ b/drivers/hv/hv.c
> @@ -15,6 +15,7 @@
>  #include <linux/hyperv.h>
>  #include <linux/random.h>
>  #include <linux/clockchips.h>
> +#include <linux/delay.h>
>  #include <linux/interrupt.h>
>  #include <clocksource/hyperv_timer.h>
>  #include <asm/mshyperv.h>
> @@ -292,12 +293,50 @@ void hv_synic_disable_regs(unsigned int cpu)
>  		disable_percpu_irq(vmbus_irq);
>  }
> 
> +#define HV_MAX_TRIES 3
> +/*
> + * Scan the event flags page of 'this' CPU looking for any bit that is set.  If we find one
> + * bit set, then wait for a few milliseconds.  Repeat these steps for a maximum of 3 times.
> + * Return 'true', if there is still any set bit after this operation; 'false', otherwise.
> + *
> + * If a bit is set, that means there is a pending channel interrupt.  The expectation is
> + * that the normal interrupt handling mechanism will find and process the channel interrupt
> + * "very soon", and in the process clear the bit.
> + */
> +static bool hv_synic_event_pending(void)
> +{
> +	struct hv_per_cpu_context *hv_cpu = this_cpu_ptr(hv_context.cpu_context);
> +	union hv_synic_event_flags *event =
> +		(union hv_synic_event_flags *)hv_cpu->synic_event_page + VMBUS_MESSAGE_SINT;
> +	unsigned long *recv_int_page = event->flags; /* assumes VMBus version >= VERSION_WIN8 */
> +	bool pending;
> +	u32 relid;
> +	int tries = 0;
> +
> +retry:
> +	pending = false;
> +	for_each_set_bit(relid, recv_int_page, HV_EVENT_FLAGS_COUNT) {
> +		/* Special case - VMBus channel protocol messages */
> +		if (relid == 0)
> +			continue;
> +		pending = true;
> +		break;
> +	}
> +	if (pending && tries++ < HV_MAX_TRIES) {
> +		usleep_range(10000, 20000);
> +		goto retry;
> +	}
> +	return pending;
> +}
> 
>  int hv_synic_cleanup(unsigned int cpu)
>  {
>  	struct vmbus_channel *channel, *sc;
>  	bool channel_found = false;
> 
> +	if (vmbus_connection.conn_state != CONNECTED)
> +		goto always_cleanup;
> +
>  	/*
>  	 * Hyper-V does not provide a way to change the connect CPU once
>  	 * it is set; we must prevent the connect CPU from going offline
> @@ -305,8 +344,7 @@ int hv_synic_cleanup(unsigned int cpu)
>  	 * path where the vmbus is already disconnected, the CPU must be
>  	 * allowed to shut down.
>  	 */
> -	if (cpu == VMBUS_CONNECT_CPU &&
> -	    vmbus_connection.conn_state == CONNECTED)
> +	if (cpu == VMBUS_CONNECT_CPU)
>  		return -EBUSY;
> 
>  	/*
> @@ -333,9 +371,21 @@ int hv_synic_cleanup(unsigned int cpu)
>  	}
>  	mutex_unlock(&vmbus_connection.channel_mutex);
> 
> -	if (channel_found && vmbus_connection.conn_state == CONNECTED)
> +	if (channel_found)
> +		return -EBUSY;
> +
> +	/*
> +	 * channel_found == false means that any channels that were previously
> +	 * assigned to the CPU have been reassigned elsewhere with a call of
> +	 * vmbus_send_modifychannel().  Scan the event flags page looking for
> +	 * bits that are set and waiting with a timeout for vmbus_chan_sched()
> +	 * to process such bits.  If bits are still set after this operation
> +	 * and VMBus is connected, fail the CPU offlining operation.
> +	 */
> +	if (vmbus_proto_version >= VERSION_WIN10_V4_1 && hv_synic_event_pending())
>  		return -EBUSY;
> 
> +always_cleanup:
>  	hv_stimer_legacy_cleanup(cpu);
> 
>  	hv_synic_disable_regs(cpu);
> --
> 2.25.1