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

[Paul Durrant <Paul.Durrant@xxxxxxxxxx>]

> -----Original Message-----
> From: qemu-devel-bounces+paul.durrant=citrix.com@xxxxxxxxxx
> [mailto:qemu-devel-bounces+paul.durrant=citrix.com@xxxxxxxxxx] On
> Behalf Of Stefano Stabellini
> Sent: 06 January 2016 12:08
> To: qemu-devel@xxxxxxxxxx
> Cc: liuling-it@xxxxxx; xen-devel@xxxxxxxxxxxxxxxxxxx; 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@xxxxxxxxxxxxx>
> 
> 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;


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