RE: [PATCH 5/7] Drivers: hv: vmbus: Ignore the offers when resuming from hibernation

From: Michael Kelley <mikelley@microsoft.com>
To: Dexuan Cui <decui@microsoft.com>,
	"linux-hyperv@vger.kernel.org" <linux-hyperv@vger.kernel.org>,
	"gregkh@linuxfoundation.org" <gregkh@linuxfoundation.org>,
	Stephen Hemminger <sthemmin@microsoft.com>,
	Sasha Levin <Alexander.Levin@microsoft.com>,
	"sashal@kernel.org" <sashal@kernel.org>,
	Haiyang Zhang <haiyangz@microsoft.com>,
	KY Srinivasan <kys@microsoft.com>,
	"tglx@linutronix.de" <tglx@linutronix.de>
Cc: "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
Subject: RE: [PATCH 5/7] Drivers: hv: vmbus: Ignore the offers when resuming from hibernation
Date: Tue, 30 Jul 2019 23:07:08 +0000	[thread overview]
Message-ID: <MWHPR21MB078481A7C7F65E0297135D41D7DC0@MWHPR21MB0784.namprd21.prod.outlook.com> (raw)
In-Reply-To: <1562650084-99874-6-git-send-email-decui@microsoft.com>

From: Dexuan Cui <decui@microsoft.com> Sent: Monday, July 8, 2019 10:29 PM
> 
> When the VM resumes, the host re-sends the offers. We should not add the
> offers to the global vmbus_connection.chn_list again.
> 
> Added some debug code, in case the host screws up the exact info related to
> the offers.
> 
> Signed-off-by: Dexuan Cui <decui@microsoft.com>
> ---
>  drivers/hv/channel_mgmt.c | 28 +++++++++++++++++++++++++++-
>  1 file changed, 27 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/hv/channel_mgmt.c b/drivers/hv/channel_mgmt.c
> index addcef5..a9aeeab 100644
> --- a/drivers/hv/channel_mgmt.c
> +++ b/drivers/hv/channel_mgmt.c
> @@ -854,12 +854,38 @@ void vmbus_initiate_unload(bool crash)
>  static void vmbus_onoffer(struct vmbus_channel_message_header *hdr)
>  {
>  	struct vmbus_channel_offer_channel *offer;
> -	struct vmbus_channel *newchannel;
> +	struct vmbus_channel *oldchannel, *newchannel;
> +	size_t offer_sz;
> 
>  	offer = (struct vmbus_channel_offer_channel *)hdr;
> 
>  	trace_vmbus_onoffer(offer);
> 
> +	mutex_lock(&vmbus_connection.channel_mutex);
> +	oldchannel = relid2channel(offer->child_relid);
> +	mutex_unlock(&vmbus_connection.channel_mutex);
> +
> +	if (oldchannel != NULL) {
> +		atomic_dec(&vmbus_connection.offer_in_progress);
> +
> +		/*
> +		 * We're resuming from hibernation: we expect the host to send
> +		 * exactly the same offers that we had before the hibernation.
> +		 */
> +		offer_sz = sizeof(*offer);
> +		if (memcmp(offer, &oldchannel->offermsg, offer_sz) == 0)
> +			return;

The offermsg contains "reserved" and "padding" fields.  Does Hyper-V
guarantee that all these fields are the same in the new offer after resuming
from hibernation?  Or should a less stringent check be made?  For example,
I could imagine a newer version of Hyper-V allowing a VM that was
hibernated on an older version to be resumed.  But one of the reserved fields
might be used in the newer version, and the comparison could fail
unnecessarily.

> +
> +		pr_err("Mismatched offer from the host (relid=%d)!\n",
> +		       offer->child_relid);
> +
> +		print_hex_dump_debug("Old vmbus offer: ", DUMP_PREFIX_OFFSET, 4,
> +				     4, &oldchannel->offermsg, offer_sz, false);
> +		print_hex_dump_debug("New vmbus offer: ", DUMP_PREFIX_OFFSET, 4,
> +				     4, offer, offer_sz, false);

The third argument to print_hex_dump() is the rowsize and is specified as must
be 16 or 32.  

> +		return;
> +	}
> +
>  	/* Allocate the channel object and save this offer. */
>  	newchannel = alloc_channel();
>  	if (!newchannel) {
> --
> 1.8.3.1