Re: [Xen-devel] pv-grub guest booting fail with recent qemu-xen

["Hao, Xudong" <xudong.hao@xxxxxxxxx>]

> -----Original Message-----
> From: Wei Liu [mailto:wei.liu2@xxxxxxxxxx]
> Sent: Monday, April 11, 2016 6:54 PM
> To: Samuel Thibault <samuel.thibault@xxxxxxxxxxxx>; Wei Liu
> <wei.liu2@xxxxxxxxxx>; Hao, Xudong <xudong.hao@xxxxxxxxx>; Konrad
> Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>; xen-devel@xxxxxxxxxxxxx;
> stefano.stabellini@xxxxxxxxxxxxx; Anthony PERARD <anthony.perard@xxxxxxxxxx>
> Subject: Re: [Xen-devel] pv-grub guest booting fail with recent qemu-xen
> 
> On Sun, Apr 10, 2016 at 10:14:46PM +0200, Samuel Thibault wrote:
> > Hello,
> >
> > > > > > +    if (prod - out_cons >= XENFB_OUT_RING_LEN) {
> > > > > > +        return;
> > > > > > +    }
> >
> > This test seems overzealous to me: AIUI, the producer can produce
> > XENFB_OUT_RING_LEN events, and thus prod - out_cons is exactly
> > XENFB_OUT_RING_LEN, i.e. there is no room left at all.
> >
> > The frontend part is:
> >
> >    while (page->out_prod - page->out_cons == XENFB_OUT_RING_LEN)
> >         schedule();
> >
> > I.e. it waits while the buffer is exactly full.
> >
> > So it seems to me the bug is at the backend side.
> >
> 
> OK, thanks for checking!
> 
> Hudong, can you try the following diff?

Yes, This patch works with testing. 

Thanks,
-Xudong

> 
> diff --git a/hw/display/xenfb.c b/hw/display/xenfb.c index 8eb3046..284b345
> 100644
> --- a/hw/display/xenfb.c
> +++ b/hw/display/xenfb.c
> @@ -789,7 +789,7 @@ 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 */
> 
> > Samuel

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