[PATCH 1/2] xenfb: use the correct condition to avoid excessive looping

[PATCH 1/2] xenfb: use the correct condition to avoid excessive looping

[Wei Liu]

In commit ac0487e1 ("xenfb.c: avoid expensive loops when prod <=
out_cons"), ">=" was used. In fact, a full ring is a legit state.
Correct the test to use ">".

Reported-by: "Hao, Xudong" <xudong.hao@intel.com>
Signed-off-by: Wei Liu <wei.liu2@citrix.com>
Tested-by: "Hao, Xudong" <xudong.hao@intel.com>
---
Cc: Stefano Stabellini <sstabellini@kernel.org>
Cc: Anthony Perard <anthony.perard@citrix.com>

Backport candidate to our own tree.
---
 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 */
-- 
2.1.4


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[PATCH 2/2] xenfb: remove out_cons in xenfb_handle_events

[Wei Liu]

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;
+    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;
-- 
2.1.4


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Re: [PATCH 2/2] xenfb: remove out_cons in xenfb_handle_events

[Andrew Cooper]

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.

~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;


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Re: [PATCH 1/2] xenfb: use the correct condition to avoid excessive looping

[Anthony PERARD]

On Tue, Apr 12, 2016 at 10:57:16AM +0100, Wei Liu wrote:
> In commit ac0487e1 ("xenfb.c: avoid expensive loops when prod <=
> out_cons"), ">=" was used. In fact, a full ring is a legit state.
> Correct the test to use ">".
> 
> Reported-by: "Hao, Xudong" <xudong.hao@intel.com>
> Signed-off-by: Wei Liu <wei.liu2@citrix.com>
> Tested-by: "Hao, Xudong" <xudong.hao@intel.com>

Acked-by: Anthony PERARD <anthony.perard@citrix.com>

> ---
> Cc: Stefano Stabellini <sstabellini@kernel.org>
> Cc: Anthony Perard <anthony.perard@citrix.com>
> 
> Backport candidate to our own tree.
> ---
>  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 */
> -- 
> 2.1.4
> 

-- 
Anthony PERARD

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Re: [PATCH 2/2] xenfb: remove out_cons in xenfb_handle_events

[Wei Liu]

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;
> 

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