Re: [Qemu-devel] [PATCH] xenfb.c: avoid expensive loops when prod <= out_cons

From: Paul Durrant <Paul.Durrant@citrix.com>
To: Stefano Stabellini <Stefano.Stabellini@citrix.com>,
	"qemu-devel@nongnu.org" <qemu-devel@nongnu.org>
Cc: "xen-devel@lists.xensource.com" <xen-devel@lists.xensource.com>,
	"liuling-it@360.cn" <liuling-it@360.cn>
Subject: Re: [Qemu-devel] [PATCH] xenfb.c: avoid expensive loops when prod <=	out_cons
Date: Wed, 6 Jan 2016 12:27:49 +0000	[thread overview]
Message-ID: <8514e238e6304412a5a94b8115d16867@AMSPEX02CL03.citrite.net> (raw)
In-Reply-To: <alpine.DEB.2.02.1601061201020.31111@kaball.uk.xensource.com>

> -----Original Message-----
> From: qemu-devel-bounces+paul.durrant=citrix.com@nongnu.org
> [mailto:qemu-devel-bounces+paul.durrant=citrix.com@nongnu.org] On
> Behalf Of Stefano Stabellini
> Sent: 06 January 2016 12:08
> To: qemu-devel@nongnu.org
> Cc: liuling-it@360.cn; xen-devel@lists.xensource.com; Stefano Stabellini
> Subject: [Qemu-devel] [PATCH] xenfb.c: avoid expensive loops when prod
> <= out_cons
> 
> If the frontend sets out_cons to a value higher than out_prod, it will
> cause xenfb_handle_events to loop about 2^32 times. Avoid that by using
> better checks at the beginning of the function.
> 

What happens when out_prod wraps?

  Paul

> Signed-off-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
> 
> diff --git a/hw/display/xenfb.c b/hw/display/xenfb.c
> index 4e2a27a..f963cf2 100644
> --- a/hw/display/xenfb.c
> +++ b/hw/display/xenfb.c
> @@ -789,10 +789,11 @@ static void xenfb_handle_events(struct XenFB
> *xenfb)
> 
>      prod = page->out_prod;
>      out_cons = page->out_cons;
> -    if (prod == out_cons)
> -	return;
> +    if (prod <= out_cons) {
> +        return;
> +    }
>      xen_rmb();		/* ensure we see ring contents up to prod */
> -    for (cons = out_cons; cons != prod; cons++) {
> +    for (cons = out_cons; cons < prod; cons++) {
>  	union xenfb_out_event *event = &XENFB_OUT_RING_REF(page,
> cons);
>          uint8_t type = event->type;
>  	int x, y, w, h;