diff mbox series

[4/4] xen: Avoid VLA

Message ID 20190617154105.32323-5-anthony.perard@citrix.com (mailing list archive)
State Superseded
Headers show
Series Fix build of Xen support + cleanup | expand

Commit Message

Anthony PERARD June 17, 2019, 3:41 p.m. UTC
Avoid using a variable length array.

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

Notes:
    Was suggested by Peter here:
    <CAFEAcA88+A2oCkQnxKDEdpmfCZSmPzWMBg01wDDV68bMZoY5Jg@mail.gmail.com>
    "should we try to stop using variable length arrays?"

 hw/i386/xen/xen-hvm.c | 11 ++++++++---
 1 file changed, 8 insertions(+), 3 deletions(-)

Comments

Paul Durrant June 17, 2019, 4:39 p.m. UTC | #1
> -----Original Message-----
> From: Anthony PERARD [mailto:anthony.perard@citrix.com]
> Sent: 17 June 2019 16:41
> To: qemu-devel@nongnu.org
> Cc: xen-devel@lists.xenproject.org; Anthony Perard <anthony.perard@citrix.com>; Stefano Stabellini
> <sstabellini@kernel.org>; Paul Durrant <Paul.Durrant@citrix.com>
> Subject: [PATCH 4/4] xen: Avoid VLA
> 
> Avoid using a variable length array.
> 
> Signed-off-by: Anthony PERARD <anthony.perard@citrix.com>
> ---
> 
> Notes:
>     Was suggested by Peter here:
>     <CAFEAcA88+A2oCkQnxKDEdpmfCZSmPzWMBg01wDDV68bMZoY5Jg@mail.gmail.com>
>     "should we try to stop using variable length arrays?"
> 
>  hw/i386/xen/xen-hvm.c | 11 ++++++++---
>  1 file changed, 8 insertions(+), 3 deletions(-)
> 
> diff --git a/hw/i386/xen/xen-hvm.c b/hw/i386/xen/xen-hvm.c
> index 725f9c2278..10d73b55b4 100644
> --- a/hw/i386/xen/xen-hvm.c
> +++ b/hw/i386/xen/xen-hvm.c
> @@ -615,7 +615,8 @@ static void xen_sync_dirty_bitmap(XenIOState *state,
>  {
>      hwaddr npages = size >> TARGET_PAGE_BITS;
>      const int width = sizeof(unsigned long) * 8;
> -    unsigned long bitmap[DIV_ROUND_UP(npages, width)];
> +    unsigned long *bitmap = NULL;
> +    size_t bitmap_size = DIV_ROUND_UP(npages, width);
>      int rc, i, j;
>      const XenPhysmap *physmap = NULL;
> 
> @@ -632,6 +633,8 @@ static void xen_sync_dirty_bitmap(XenIOState *state,
>          return;
>      }
> 
> +    bitmap = g_new0(unsigned long, bitmap_size);
> +

How hot is this function? It looks (unsurprisingly) like the section size determines the map size so I wonder whether it can instead be allocated once when the section is added?

  Paul

>      rc = xen_track_dirty_vram(xen_domid, start_addr >> TARGET_PAGE_BITS,
>                                npages, bitmap);
>      if (rc < 0) {
> @@ -644,10 +647,10 @@ static void xen_sync_dirty_bitmap(XenIOState *state,
>                      ", 0x" TARGET_FMT_plx "): %s\n",
>                      start_addr, start_addr + size, strerror(errno));
>          }
> -        return;
> +        goto out;
>      }
> 
> -    for (i = 0; i < ARRAY_SIZE(bitmap); i++) {
> +    for (i = 0; i < bitmap_size; i++) {
>          unsigned long map = bitmap[i];
>          while (map != 0) {
>              j = ctzl(map);
> @@ -657,6 +660,8 @@ static void xen_sync_dirty_bitmap(XenIOState *state,
>                                      TARGET_PAGE_SIZE);
>          };
>      }
> +out:
> +    g_free(bitmap);
>  }
> 
>  static void xen_log_start(MemoryListener *listener,
> --
> Anthony PERARD
Anthony PERARD June 17, 2019, 5:36 p.m. UTC | #2
On Mon, Jun 17, 2019 at 05:39:09PM +0100, Paul Durrant wrote:
> > @@ -632,6 +633,8 @@ static void xen_sync_dirty_bitmap(XenIOState *state,
> >          return;
> >      }
> > 
> > +    bitmap = g_new0(unsigned long, bitmap_size);
> > +
> 
> How hot is this function? It looks (unsurprisingly) like the section
> size determines the map size so I wonder whether it can instead be
> allocated once when the section is added?

I think we can store the bitmap buffer into the `state' where
`log_for_dirtybit' is already present, and free the bitmap when
`log_for_dirtybit' is cleared.

Thanks,
Paul Durrant June 18, 2019, 7:50 a.m. UTC | #3
> -----Original Message-----
> From: Anthony PERARD [mailto:anthony.perard@citrix.com]
> Sent: 17 June 2019 18:37
> To: Paul Durrant <Paul.Durrant@citrix.com>
> Cc: qemu-devel@nongnu.org; xen-devel@lists.xenproject.org; Stefano Stabellini <sstabellini@kernel.org>
> Subject: Re: [PATCH 4/4] xen: Avoid VLA
> 
> On Mon, Jun 17, 2019 at 05:39:09PM +0100, Paul Durrant wrote:
> > > @@ -632,6 +633,8 @@ static void xen_sync_dirty_bitmap(XenIOState *state,
> > >          return;
> > >      }
> > >
> > > +    bitmap = g_new0(unsigned long, bitmap_size);
> > > +
> >
> > How hot is this function? It looks (unsurprisingly) like the section
> > size determines the map size so I wonder whether it can instead be
> > allocated once when the section is added?
> 
> I think we can store the bitmap buffer into the `state' where
> `log_for_dirtybit' is already present, and free the bitmap when
> `log_for_dirtybit' is cleared.

That sounds better :-)

Cheers,

  Paul

> 
> Thanks,
> 
> --
> Anthony PERARD
diff mbox series

Patch

diff --git a/hw/i386/xen/xen-hvm.c b/hw/i386/xen/xen-hvm.c
index 725f9c2278..10d73b55b4 100644
--- a/hw/i386/xen/xen-hvm.c
+++ b/hw/i386/xen/xen-hvm.c
@@ -615,7 +615,8 @@  static void xen_sync_dirty_bitmap(XenIOState *state,
 {
     hwaddr npages = size >> TARGET_PAGE_BITS;
     const int width = sizeof(unsigned long) * 8;
-    unsigned long bitmap[DIV_ROUND_UP(npages, width)];
+    unsigned long *bitmap = NULL;
+    size_t bitmap_size = DIV_ROUND_UP(npages, width);
     int rc, i, j;
     const XenPhysmap *physmap = NULL;
 
@@ -632,6 +633,8 @@  static void xen_sync_dirty_bitmap(XenIOState *state,
         return;
     }
 
+    bitmap = g_new0(unsigned long, bitmap_size);
+
     rc = xen_track_dirty_vram(xen_domid, start_addr >> TARGET_PAGE_BITS,
                               npages, bitmap);
     if (rc < 0) {
@@ -644,10 +647,10 @@  static void xen_sync_dirty_bitmap(XenIOState *state,
                     ", 0x" TARGET_FMT_plx "): %s\n",
                     start_addr, start_addr + size, strerror(errno));
         }
-        return;
+        goto out;
     }
 
-    for (i = 0; i < ARRAY_SIZE(bitmap); i++) {
+    for (i = 0; i < bitmap_size; i++) {
         unsigned long map = bitmap[i];
         while (map != 0) {
             j = ctzl(map);
@@ -657,6 +660,8 @@  static void xen_sync_dirty_bitmap(XenIOState *state,
                                     TARGET_PAGE_SIZE);
         };
     }
+out:
+    g_free(bitmap);
 }
 
 static void xen_log_start(MemoryListener *listener,