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SubjectRe: [Xen-devel] [PATCH] xen: fix ring resize of /dev/evtchn
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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@oracle.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

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