Re: [Xen-devel] [PATCH 4/4] xen: Avoid VLA

[Paul Durrant <Paul.Durrant@citrix.com>]

> -----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


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