Re: [Xen-devel] [Qemu-devel] [PATCH v2 4/4] xen: Avoid VLA

[Philippe Mathieu-Daudé <philmd@xxxxxxxxxx>]

On 6/18/19 1:23 PM, Anthony PERARD wrote:
> Avoid using a variable length array.
> 
> We allocate the `dirty_bitmap' buffer only once when we start tracking
> for dirty bits.
> 

Suggested-by: Paul Durrant <Paul.Durrant@xxxxxxxxxx>

Reviewed-by: Philippe Mathieu-Daudé <philmd@xxxxxxxxxx>

> Signed-off-by: Anthony PERARD <anthony.perard@xxxxxxxxxx>
> ---
> 
> Notes:
>     v2:
>     - Allocate the bitmap buffer only once when we start tracking dirty bits.
>       (instead of at every function call)
>     
>     Was suggested by Peter here:
>     <CAFEAcA88+A2oCkQnxKDEdpmfCZSmPzWMBg01wDDV68bMZoY5Jg@xxxxxxxxxxxxxx>
>     "should we try to stop using variable length arrays?"
> 
>  hw/i386/xen/xen-hvm.c | 15 +++++++++++----
>  1 file changed, 11 insertions(+), 4 deletions(-)
> 
> diff --git a/hw/i386/xen/xen-hvm.c b/hw/i386/xen/xen-hvm.c
> index ae3deb4ef3..469f1260a4 100644
> --- a/hw/i386/xen/xen-hvm.c
> +++ b/hw/i386/xen/xen-hvm.c
> @@ -119,6 +119,8 @@ typedef struct XenIOState {
>      DeviceListener device_listener;
>      hwaddr free_phys_offset;
>      const XenPhysmap *log_for_dirtybit;
> +    /* Buffer used by xen_sync_dirty_bitmap */
> +    unsigned long *dirty_bitmap;
>  
>      Notifier exit;
>      Notifier suspend;
> @@ -464,6 +466,8 @@ static int xen_remove_from_physmap(XenIOState *state,
>      QLIST_REMOVE(physmap, list);
>      if (state->log_for_dirtybit == physmap) {
>          state->log_for_dirtybit = NULL;
> +        g_free(state->dirty_bitmap);
> +        state->dirty_bitmap = NULL;
>      }
>      g_free(physmap);
>  
> @@ -614,7 +618,7 @@ 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)];
> +    size_t bitmap_size = DIV_ROUND_UP(npages, width);
>      int rc, i, j;
>      const XenPhysmap *physmap = NULL;
>  
> @@ -626,13 +630,14 @@ static void xen_sync_dirty_bitmap(XenIOState *state,
>  
>      if (state->log_for_dirtybit == NULL) {
>          state->log_for_dirtybit = physmap;
> +        state->dirty_bitmap = g_new(unsigned long, bitmap_size);
>      } else if (state->log_for_dirtybit != physmap) {
>          /* Only one range for dirty bitmap can be tracked. */
>          return;
>      }
>  
>      rc = xen_track_dirty_vram(xen_domid, start_addr >> TARGET_PAGE_BITS,
> -                              npages, bitmap);
> +                              npages, state->dirty_bitmap);
>      if (rc < 0) {
>  #ifndef ENODATA
>  #define ENODATA  ENOENT
> @@ -646,8 +651,8 @@ static void xen_sync_dirty_bitmap(XenIOState *state,
>          return;
>      }
>  
> -    for (i = 0; i < ARRAY_SIZE(bitmap); i++) {
> -        unsigned long map = bitmap[i];
> +    for (i = 0; i < bitmap_size; i++) {
> +        unsigned long map = state->dirty_bitmap[i];
>          while (map != 0) {
>              j = ctzl(map);
>              map &= ~(1ul << j);
> @@ -677,6 +682,8 @@ static void xen_log_stop(MemoryListener *listener, 
> MemoryRegionSection *section,
>  
>      if (old & ~new & (1 << DIRTY_MEMORY_VGA)) {
>          state->log_for_dirtybit = NULL;
> +        g_free(state->dirty_bitmap);
> +        state->dirty_bitmap = NULL;
>          /* Disable dirty bit tracking */
>          xen_track_dirty_vram(xen_domid, 0, 0, NULL);
>      }
> 

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