linux-kernel - Re: [RFC PATCH 1/4] Drivers: hv: vmbus: Remove special code for unsolicited messages

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Message-ID: <20220407025425.GA32474@anparri>
Date:   Thu, 7 Apr 2022 04:54:25 +0200
From:   Andrea Parri <parri.andrea@...il.com>
To:     KY Srinivasan <kys@...rosoft.com>,
        Haiyang Zhang <haiyangz@...rosoft.com>,
        Stephen Hemminger <sthemmin@...rosoft.com>,
        Wei Liu <wei.liu@...nel.org>, Dexuan Cui <decui@...rosoft.com>,
        Michael Kelley <mikelley@...rosoft.com>,
        Wei Hu <weh@...rosoft.com>,
        Lorenzo Pieralisi <lorenzo.pieralisi@....com>,
        Rob Herring <robh@...nel.org>,
        Krzysztof Wilczynski <kw@...ux.com>,
        Bjorn Helgaas <bhelgaas@...gle.com>
Cc:     linux-hyperv@...r.kernel.org, linux-pci@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH 1/4] Drivers: hv: vmbus: Remove special code for
 unsolicited messages

On Mon, Mar 28, 2022 at 04:42:41PM +0200, Andrea Parri (Microsoft) wrote:
> vmbus_requestor has included code for handling unsolicited messages
> since its introduction with commit e8b7db38449ac ("Drivers: hv: vmbus:
> Add vmbus_requestor data structure for VMBus hardening"); such code was
> motivated by the early use of vmbus_requestor from storvsc.  Since
> storvsc moved to a tag-based mechanism to generate/retrieve request IDs
> with commit bf5fd8cae3c8f ("scsi: storvsc: Use blk_mq_unique_tag() to
> generate requestIDs"), the special handling of unsolicited messages in
> vmbus_requestor is not useful and can be removed.

As it turns out, this is not quite right.  In particular...


> @@ -1243,11 +1243,7 @@ u64 vmbus_next_request_id(struct vmbus_channel *channel, u64 rqst_addr)
>  
>  	spin_unlock_irqrestore(&rqstor->req_lock, flags);
>  
> -	/*
> -	 * Cannot return an ID of 0, which is reserved for an unsolicited
> -	 * message from Hyper-V.
> -	 */
> -	return current_id + 1;
> +	return current_id;

Hyper-V treats requests with ID of 0 as "non-transactional" and it does
not respond to such requests.


> @@ -1268,15 +1264,8 @@ u64 vmbus_request_addr(struct vmbus_channel *channel, u64 trans_id)
>  	if (!channel->rqstor_size)
>  		return VMBUS_NO_RQSTOR;
>  
> -	/* Hyper-V can send an unsolicited message with ID of 0 */
> -	if (!trans_id)
> -		return trans_id;

This remains problematic: I will elaborate and propose some solution in
the next iteration (to be sent shortly).

Thanks,
  Andrea

> -
>  	spin_lock_irqsave(&rqstor->req_lock, flags);
>  
> -	/* Data corresponding to trans_id is stored at trans_id - 1 */
> -	trans_id--;
> -
>  	/* Invalid trans_id */
>  	if (trans_id >= rqstor->size || !test_bit(trans_id, rqstor->req_bitmap)) {
>  		spin_unlock_irqrestore(&rqstor->req_lock, flags);