linux-kernel - Re: [Xen-devel] [PATCH v2 2/2] block/xen-blkfront: Handle non-indirect grant with 64KB pages

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Message-ID: <5644D494.3030700@citrix.com>
Date:	Thu, 12 Nov 2015 18:04:04 +0000
From:	Julien Grall <julien.grall@...rix.com>
To:	Roger Pau Monné <roger.pau@...rix.com>,
	<xen-devel@...ts.xenproject.org>, <linux-kernel@...r.kernel.org>
CC:	Boris Ostrovsky <boris.ostrovsky@...cle.com>,
	David Vrabel <david.vrabel@...rix.com>
Subject: Re: [Xen-devel] [PATCH v2 2/2] block/xen-blkfront: Handle non-indirect
 grant with 64KB pages

On 12/11/15 17:51, Roger Pau Monné wrote:
> El 12/11/15 a les 18.30, Julien Grall ha escrit:
>> Hi,
>>
>> On 12/11/15 16:40, Roger Pau Monné wrote:
>>>> [1] http://lists.xen.org/archives/html/xen-devel/2015-08/msg02200.html
>>>>
>>>> Signed-off-by: Julien Grall <julien.grall@...rix.com>
>>>
>>> LGTM, only a couple of typos and a simplification:
>>>
>>> Signed-off-by: Roger Pau Monné <roger.pau@...rix.com>
>>
>> Do you mean Acked-by? ;)
> 
> Yes, I also had problems with smtp, so I thought this one was actually
> not sent. You have another one with a proper Ack :).
> 
>>>> +
>>>> +	/* The I/O request may be split in two */
>>>> +	if (unlikely(s->associated_id != NO_ASSOCIATED_ID)) {
>>>> +		struct blk_shadow *s2 = &info->shadow[s->associated_id];
>>>> +
>>>> +		/* Keep the status of the current response in shadow */
>>>> +		s->status = (bret->status == BLKIF_RSP_OKAY) ?
>>>> +			REQ_DONE : REQ_FAIL;
>>>> +
>>>> +		/* Wait the second response if not yet here */
>>>> +		if (s2->status == REQ_WAITING)
>>>> +			return 0;
>>>> +
>>>> +		/*
>>>> +		 * The status of the current response will be used in
>>>> +		 * order to know if the request has failed.
>>>> +		 * Update the current response status only if has not
>>>> +		 * failed.
>>>> +		 */
>>>> +		if (bret->status == BLKIF_RSP_OKAY && s2->status == REQ_FAIL)
>>>
>>> This could be simplified by only checking if s2->status == REQ_FAIL.
>>
>> I didn't do it because bret->status may be different than
>> BLKIF_RSP_ERROR (for instance BLKIF_RSP_EOPNOTSUPP).
> 
> I think this is not actually possible in practice, but what if
> bret->status == BLKIF_RSP_OKAY and the bret from s2 actually had
> BLKIF_RSP_EOPNOTSUPP, wouldn't we loose the EOPNOTSUPP by
> unconditionally setting BLKIF_RSP_ERROR?

No because EOPNOTSUPP are used when an operation is not supported. As
the 2 ring request is coming from the same I/O request, it will always
have the same operation.

So if one get EOPNOTSUPP the other will get too.

> 
> Should s->status be able to store all the possible return codes from the
> response (OK/ERROR/NOTSUPP)?

That could would work. However, how do you decide which will be the
final status?

Regards,

-- 
Julien Grall
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