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Date: Mon, 18 Apr 2016 15:40:15 +0100
From: Wei Liu <wei.liu2@citrix.com>
Message-ID: <20160418144015.GA26228@citrix.com>
References: <1460983308-28000-1-git-send-email-samuel.thibault@ens-lyon.org>
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In-Reply-To: <1460983308-28000-1-git-send-email-samuel.thibault@ens-lyon.org>
Subject: Re: [Qemu-devel] [PATCH] xenfb: Fix overzealous ring test
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To: Samuel Thibault <samuel.thibault@ens-lyon.org>
Cc: qemu-devel@nongnu.org, Wei Liu <wei.liu2@citrix.com>, Stefano Stabellini <sstabellini@kernel.org>, Anthony Perard <anthony.perard@citrix.com>, xen-devel@lists.xensource.com

On Mon, Apr 18, 2016 at 02:41:48PM +0200, Samuel Thibault wrote:
> From: Wei Liu <wei.liu2@citrix.com>
> 
> pv-grub booting got broken with recent qemu-xen, due to
> ac0487e1d2ae811cd4d035741a109a4ecfb013f1 ('xenfb.c: avoid expensive loops
> when prod <= out_cons')
> 
> prod - out_cons can actually be XENFB_OUT_RING_LEN when the ring is exactly
> full, this is a normal condition and should not be excluded.
> 
> From: Wei Liu <wei.liu2@citrix.com>
> Signed-off-by: Samuel Thibault <samuel.thibault@ens-lyon.org>
> Tested-by: Hao Xudong <xudong.hao@intel.com>

Hi Samuel,

This patch is already queued by Stefano. After it has been applied
upstream we will backport it to our own tree.

Wei.

> ---
>  hw/display/xenfb.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/hw/display/xenfb.c b/hw/display/xenfb.c
> index 40b096a..9866dfd 100644
> --- a/hw/display/xenfb.c
> +++ b/hw/display/xenfb.c
> @@ -775,7 +775,7 @@ static void xenfb_handle_events(struct XenFB *xenfb)
>  
>      prod = page->out_prod;
>      out_cons = page->out_cons;
> -    if (prod - out_cons >= XENFB_OUT_RING_LEN) {
> +    if (prod - out_cons > XENFB_OUT_RING_LEN) {
>          return;
>      }
>      xen_rmb();		/* ensure we see ring contents up to prod */
> -- 
> 2.8.0.rc3
>