linux-kernel - Re: [PATCH v4] drivers/tpm: add xen tpmfront interface

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Message-ID: <51F00A4A.5060005@citrix.com>
Date:	Wed, 24 Jul 2013 18:09:30 +0100
From:	David Vrabel <david.vrabel@...rix.com>
To:	Daniel De Graaf <dgdegra@...ho.nsa.gov>
CC:	<konrad.wilk@...cle.com>, <tpmdd-devel@...ts.sourceforge.net>,
	<linux-kernel@...r.kernel.org>, <leosilva@...ux.vnet.ibm.com>,
	<shpedoikal@...il.com>, <xen-devel@...ts.xen.org>,
	<mail@...jiv.net>, <adlai@...ux.vnet.ibm.com>, <tpmdd@...rix.com>,
	<PeterHuewe@....de>
Subject: Re: [PATCH v4] drivers/tpm: add xen tpmfront interface

On 22/07/13 17:20, Daniel De Graaf wrote:
> On 07/22/2013 11:18 AM, David Vrabel wrote:
>> On 01/07/13 22:34, Daniel De Graaf wrote:
>>> This is a complete rewrite of the Xen TPM frontend driver, taking
>>> advantage of a simplified frontend/backend interface and adding support
>>> for cancellation and timeouts.  The backend for this driver is provided
>>> by a vTPM stub domain using the interface in Xen 4.3.
>> [...]
>>> --- /dev/null
>>> +++ b/Documentation/xen-tpmfront.txt
>>
>> Suggest putting this in Documentation/tpm/.
> 
> OK.
> 
>>> --- /dev/null
>>> +++ b/drivers/char/tpm/xen-tpmfront.c
>> [...]
>>> +static void backend_changed(struct xenbus_device *dev,
>>> +        enum xenbus_state backend_state)
>>> +{
>>> +    int val;
>>
>> Hrm. I don't like how every front/back pair invents their own variation
>> of the state machine.
>>
>> Please document the front and back state machines in
>> xen/include/public/io/tpmif.h (and the correspoding copy in Linux).
> 
> Is there a standard state machine that would allow devices to avoid
> inventing their own?

That would be nice wouldn't it?  But, no, there isn't one -- even
netfront and blkfront are different.

> Otherwise, this is what I plan to add to the header:
> /*
>  * Xenbus state machine
>  *
>  * Device open:
>  *   1. Both ends start in XenbusStateInitialising
>  *   2. Backend transitions to InitWait (frontend does not wait on this
> step)
>  *   3. Frontend populates ring-ref, event-channel, feature-protocol-v2
>  *   4. Frontend transitions to Initialised
>  *   5. Backend maps grant and event channel, verifies feature-protocol-v2
>  *   6. Backend transitions to Connected
>  *   7. Frontend verifies feature-protocol-v2, transitions to Connected
>  *
>  * Device close:
>  *   1. State is changed to XenbusStateClosing
>  *   2. Frontend transitions to Closed
>  *   3. Backend unmaps grant and event, changes state to InitWait
>  */

That's good, thanks.

David
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