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

[Wei Liu <wei.liu2@xxxxxxxxxx>]

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?

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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