Re: [Qemu-devel] [Xen-devel] [PATCH 2/2] xenfb: remove out_cons in xenfb_handle_events

[Wei Liu <wei.liu2@citrix.com>]

On Tue, Apr 12, 2016 at 10:59:53AM +0100, Andrew Cooper wrote:
> On 12/04/16 10:57, Wei Liu wrote:
> > The variable out_cons was only used to temporarily hold the consumer
> > index. Use cons directly to simplify code a bit.
> >
> > No functional change introduced.
> >
> > Signed-off-by: Wei Liu <wei.liu2@citrix.com>
> > ---
> > Cc: Stefano Stabellini <sstabellini@kernel.org>
> > Cc: Anthony Perard <anthony.perard@citrix.com>
> > ---
> >  hw/display/xenfb.c | 8 ++++----
> >  1 file changed, 4 insertions(+), 4 deletions(-)
> >
> > diff --git a/hw/display/xenfb.c b/hw/display/xenfb.c
> > index 9866dfd..78a8dcd 100644
> > --- a/hw/display/xenfb.c
> > +++ b/hw/display/xenfb.c
> > @@ -770,16 +770,16 @@ static void xenfb_invalidate(void *opaque)
> >  
> >  static void xenfb_handle_events(struct XenFB *xenfb)
> >  {
> > -    uint32_t prod, cons, out_cons;
> > +    uint32_t prod, cons;
> >      struct xenfb_page *page = xenfb->c.page;
> >  
> >      prod = page->out_prod;
> > -    out_cons = page->out_cons;
> > -    if (prod - out_cons > XENFB_OUT_RING_LEN) {
> > +    cons = page->out_cons;
> 
> You need the xen_rmb() before the first use of prod or cons.
> 

Good point. I overlooked that.

The change should belong to either previous patch or a new patch though.

Wei.

> ~Andrew
> 
> > +    if (prod - cons > XENFB_OUT_RING_LEN) {
> >          return;
> >      }
> >      xen_rmb();		/* ensure we see ring contents up to prod */
> > -    for (cons = out_cons; cons != prod; cons++) {
> > +    for ( ; cons != prod; cons++) {
> >  	union xenfb_out_event *event = &XENFB_OUT_RING_REF(page, cons);
> >          uint8_t type = event->type;
> >  	int x, y, w, h;
>