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

Thanks.

David

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