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From:   Vitaly Kuznetsov <vkuznets@redhat.com>
To:     "Andrea Parri \(Microsoft\)" <parri.andrea@gmail.com>,
        Dexuan Cui <decui@microsoft.com>
Cc:     "K . Y . Srinivasan" <kys@microsoft.com>,
        Haiyang Zhang <haiyangz@microsoft.com>,
        Stephen Hemminger <sthemmin@microsoft.com>,
        Wei Liu <wei.liu@kernel.org>, linux-hyperv@vger.kernel.org,
        Michael Kelley <mikelley@microsoft.com>,
        Boqun Feng <boqun.feng@gmail.com>, linux-kernel@vger.kernel.org
Subject: Re: [RFC PATCH 01/11] Drivers: hv: vmbus: Always handle the VMBus messages on CPU0
In-Reply-To: <20200325225505.23998-2-parri.andrea@gmail.com>
References: <20200325225505.23998-1-parri.andrea@gmail.com> <20200325225505.23998-2-parri.andrea@gmail.com>
Date:   Thu, 26 Mar 2020 15:05:17 +0100
Message-ID: <874kub5i02.fsf@vitty.brq.redhat.com>
MIME-Version: 1.0
Content-Type: text/plain
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

"Andrea Parri (Microsoft)" <parri.andrea@gmail.com> writes:

> A Linux guest have to pick a "connect CPU" to communicate with the
> Hyper-V host.  This CPU can not be taken offline because Hyper-V does
> not provide a way to change that CPU assignment.
>
> Current code sets the connect CPU to whatever CPU ends up running the
> function vmbus_negotiate_version(), and this will generate problems if
> that CPU is taken offine.
>
> Establish CPU0 as the connect CPU, and add logics to prevents the
> connect CPU from being taken offline.   We could pick some other CPU,
> and we could pick that "other CPU" dynamically if there was a reason to
> do so at some point in the future.  But for now, #defining the connect
> CPU to 0 is the most straightforward and least complex solution.
>
> While on this, add inline comments explaining "why" offer and rescind
> messages should not be handled by a same serialized work queue.
>
> Suggested-by: Dexuan Cui <decui@microsoft.com>
> Signed-off-by: Andrea Parri (Microsoft) <parri.andrea@gmail.com>
> ---
>  drivers/hv/connection.c   | 20 +-------------------
>  drivers/hv/hv.c           |  7 +++++++
>  drivers/hv/hyperv_vmbus.h | 11 ++++++-----
>  drivers/hv/vmbus_drv.c    | 20 +++++++++++++++++---
>  4 files changed, 31 insertions(+), 27 deletions(-)
>
> diff --git a/drivers/hv/connection.c b/drivers/hv/connection.c
> index 74e77de89b4f3..f4bd306d2cef9 100644
> --- a/drivers/hv/connection.c
> +++ b/drivers/hv/connection.c
> @@ -69,7 +69,6 @@ MODULE_PARM_DESC(max_version,
>  int vmbus_negotiate_version(struct vmbus_channel_msginfo *msginfo, u32 version)
>  {
>  	int ret = 0;
> -	unsigned int cur_cpu;
>  	struct vmbus_channel_initiate_contact *msg;
>  	unsigned long flags;
>  
> @@ -102,24 +101,7 @@ int vmbus_negotiate_version(struct vmbus_channel_msginfo *msginfo, u32 version)
>  
>  	msg->monitor_page1 = virt_to_phys(vmbus_connection.monitor_pages[0]);
>  	msg->monitor_page2 = virt_to_phys(vmbus_connection.monitor_pages[1]);
> -	/*
> -	 * We want all channel messages to be delivered on CPU 0.
> -	 * This has been the behavior pre-win8. This is not
> -	 * perf issue and having all channel messages delivered on CPU 0
> -	 * would be ok.
> -	 * For post win8 hosts, we support receiving channel messagges on
> -	 * all the CPUs. This is needed for kexec to work correctly where
> -	 * the CPU attempting to connect may not be CPU 0.
> -	 */
> -	if (version >= VERSION_WIN8_1) {
> -		cur_cpu = get_cpu();
> -		msg->target_vcpu = hv_cpu_number_to_vp_number(cur_cpu);
> -		vmbus_connection.connect_cpu = cur_cpu;
> -		put_cpu();
> -	} else {
> -		msg->target_vcpu = 0;
> -		vmbus_connection.connect_cpu = 0;
> -	}
> +	msg->target_vcpu = hv_cpu_number_to_vp_number(VMBUS_CONNECT_CPU);
>  
>  	/*
>  	 * Add to list before we send the request since we may
> diff --git a/drivers/hv/hv.c b/drivers/hv/hv.c
> index 6098e0cbdb4b0..e2b3310454640 100644
> --- a/drivers/hv/hv.c
> +++ b/drivers/hv/hv.c
> @@ -249,6 +249,13 @@ int hv_synic_cleanup(unsigned int cpu)
>  	bool channel_found = false;
>  	unsigned long flags;
>  
> +	/*
> +	 * 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.
> +	 */
> +	if (cpu == VMBUS_CONNECT_CPU)
> +		return -EBUSY;
> +
>  	/*
>  	 * Search for channels which are bound to the CPU we're about to
>  	 * cleanup. In case we find one and vmbus is still connected we need to
> diff --git a/drivers/hv/hyperv_vmbus.h b/drivers/hv/hyperv_vmbus.h
> index 70b30e223a578..67fb1edcbf527 100644
> --- a/drivers/hv/hyperv_vmbus.h
> +++ b/drivers/hv/hyperv_vmbus.h
> @@ -212,12 +212,13 @@ enum vmbus_connect_state {
>  
>  #define MAX_SIZE_CHANNEL_MESSAGE	HV_MESSAGE_PAYLOAD_BYTE_COUNT
>  
> -struct vmbus_connection {
> -	/*
> -	 * CPU on which the initial host contact was made.
> -	 */
> -	int connect_cpu;
> +/*
> + * The CPU that Hyper-V will interrupt for VMBUS messages, such as
> + * CHANNELMSG_OFFERCHANNEL and CHANNELMSG_RESCIND_CHANNELOFFER.
> + */
> +#define VMBUS_CONNECT_CPU	0
>  
> +struct vmbus_connection {
>  	u32 msg_conn_id;
>  
>  	atomic_t offer_in_progress;
> diff --git a/drivers/hv/vmbus_drv.c b/drivers/hv/vmbus_drv.c
> index 029378c27421d..7600615e13754 100644
> --- a/drivers/hv/vmbus_drv.c
> +++ b/drivers/hv/vmbus_drv.c
> @@ -1056,14 +1056,28 @@ void vmbus_on_msg_dpc(unsigned long data)
>  			/*
>  			 * If we are handling the rescind message;
>  			 * schedule the work on the global work queue.
> +			 *
> +			 * The OFFER message and the RESCIND message should
> +			 * not be handled by the same serialized work queue,
> +			 * because the OFFER handler may call vmbus_open(),
> +			 * which tries to open the channel by sending an
> +			 * OPEN_CHANNEL message to the host and waits for
> +			 * the host's response; however, if the host has
> +			 * rescinded the channel before it receives the
> +			 * OPEN_CHANNEL message, the host just silently
> +			 * ignores the OPEN_CHANNEL message; as a result,
> +			 * the guest's OFFER handler hangs for ever, if we
> +			 * handle the RESCIND message in the same serialized
> +			 * work queue: the RESCIND handler can not start to
> +			 * run before the OFFER handler finishes.
>  			 */
> -			schedule_work_on(vmbus_connection.connect_cpu,
> +			schedule_work_on(VMBUS_CONNECT_CPU,
>  					 &ctx->work);
>  			break;
>  
>  		case CHANNELMSG_OFFERCHANNEL:
>  			atomic_inc(&vmbus_connection.offer_in_progress);
> -			queue_work_on(vmbus_connection.connect_cpu,
> +			queue_work_on(VMBUS_CONNECT_CPU,
>  				      vmbus_connection.work_queue,
>  				      &ctx->work);
>  			break;
> @@ -1110,7 +1124,7 @@ static void vmbus_force_channel_rescinded(struct vmbus_channel *channel)
>  
>  	INIT_WORK(&ctx->work, vmbus_onmessage_work);
>  
> -	queue_work_on(vmbus_connection.connect_cpu,
> +	queue_work_on(VMBUS_CONNECT_CPU,
>  		      vmbus_connection.work_queue,
>  		      &ctx->work);
>  }

I tried to refresh my memory on why 'connect_cpu' was introduced and it
all comes down to the following commit:

commit 7268644734f6a300353a4c4ff8bf3e013ba80f89
Author: Alex Ng <alexng@microsoft.com>
Date:   Fri Feb 26 15:13:22 2016 -0800

    Drivers: hv: vmbus: Support kexec on ws2012 r2 and above

which for some unknown reason kept hardcoding '0' for pre-win2012-r2 (
hv_context.vp_index[smp_processor_id()] in all cases would do exactly
the same I guess ). Later, 'connect_cpu' appeared just to remember our
choice, I can't see why we didn't go with CPU0 for simplicity.

Reviewed-by: Vitaly Kuznetsov <vkuznets@redhat.com>

-- 
Vitaly