[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]
Message-ID: <5729F984.4040807@citrix.com>
Date: Wed, 4 May 2016 14:30:44 +0100
From: David Vrabel <david.vrabel@...rix.com>
To: Jan Beulich <JBeulich@...e.com>,
David Vrabel <david.vrabel@...rix.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 14:02, 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.
Thanks.
David
Powered by blists - more mailing lists