linux-kernel - Re: [Xen-devel] [PATCH] xen: fix ring resize of /dev/evtchn

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Date:	Wed, 04 May 2016 09:42:04 -0600
From:	"Jan Beulich" <JBeulich@...e.com>
To:	"David Vrabel" <david.vrabel@...rix.com>
Cc:	"xen-devel" <xen-devel@...ts.xenproject.org>,
	"Boris Ostrovsky" <boris.ostrovsky@...cle.com>,
	"Juergen Gross" <JGross@...e.com>, <linux-kernel@...r.kernel.org>
Subject: Re: [Xen-devel] [PATCH] xen: fix ring resize of /dev/evtchn

>>> On 04.05.16 at 17:34, <david.vrabel@...rix.com> wrote:
> On 04/05/16 14:30, David Vrabel wrote:
>> On 04/05/16 14:02, Jan Beulich wrote:
>>> The copying of ring data was wrong for two cases: For a full ring
>>> nothing got copied at all (as in that case the canonicalized producer
>>> and consumer indexes are identical). And in case one or both of the
>>> canonicalized (after the resize) indexes would point into the second
>>> half of the buffer, the copied data ended up in the wrong (free) part
>>> of the new buffer. In both cases uninitialized data would get passed
>>> back to the caller.
>>>
>>> Fix this by simply copying the old ring contents twice: Once to the
>>> low half of the new buffer, and a second time to the high half.
>>>
>>> This addresses the inability to boot a HVM guest with 64 or more
>>> vCPU-s, which was reported by Konrad Rzeszutek Wilk
>>> <konrad.wilk@...cle.com>.
>> [...]
>> 
>> Can you include the commit that introduced this regression and which
>> kernel versions it affects as this is a stable candidate.
>> 
>>> @@ -344,22 +343,13 @@ static int evtchn_resize_ring(struct per
>>>  	spin_lock_irq(&u->ring_prod_lock);
>>>  
>>>  	/*
>>> -	 * Copy the old ring contents to the new ring.
>>> -	 *
>>> -	 * If the ring contents crosses the end of the current ring,
>>> -	 * it needs to be copied in two chunks.
>>> -	 *
>>> -	 * +---------+    +------------------+
>>> -	 * |34567  12| -> |       1234567    |
>>> -	 * +-----p-c-+    +------------------+
>>> +	 * Copy the old ring contents to the new ring. To take care of
>>> +	 * wrapping, a full ring, and the new canonicalized index pointing
>>> +	 * into the second half, simply copy the old contents twice.
>> 
>> Could you keep the ascii art?
>> 
>> e.g.,
>> 
>>  * +---------+    +------------------+
>>  * |34567  12| -> |34567  1234567  12|
>>  * +-----p-c-+    +-------c------p---+
>> 
>> So it is obvious that the double copy does the right thing.
> 
> Never mind, I wanted to send a pull request so I've fixed this up myself.

Oh, sorry, I had it ready but didn't want to send a v2 a few minutes
after the v1. Thanks for taking care of it!

Jan