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

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Message-ID: <572A1698.2010907@citrix.com>
Date:	Wed, 4 May 2016 16:34:48 +0100
From:	David Vrabel <david.vrabel@...rix.com>
To:	David Vrabel <david.vrabel@...rix.com>,
	Jan Beulich <JBeulich@...e.com>,
	Boris Ostrovsky <boris.ostrovsky@...cle.com>,
	Juergen Gross <JGross@...e.com>
CC:	xen-devel <xen-devel@...ts.xenproject.org>,
	<linux-kernel@...r.kernel.org>
Subject: Re: [Xen-devel] [PATCH] xen: fix ring resize of /dev/evtchn

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.

David