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

Anthony PERARD posted 4 patches 6 years, 7 months ago
There is a newer version of this series
[Xen-devel] [PATCH v2 4/4] xen: Avoid VLA
Posted by Anthony PERARD 6 years, 7 months ago 
Avoid using a variable length array.

We allocate the `dirty_bitmap' buffer only once when we start tracking
for dirty bits.

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

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@mail.gmail.com>
    "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);
     }
-- 
Anthony PERARD


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Re: [Xen-devel] [PATCH v2 4/4] xen: Avoid VLA
Posted by Paul Durrant 6 years, 7 months ago 
> -----Original Message-----
> From: Anthony PERARD [mailto:anthony.perard@citrix.com]
> Sent: 18 June 2019 12:24
> To: qemu-devel@nongnu.org
> Cc: Anthony Perard <anthony.perard@citrix.com>; Paul Durrant <Paul.Durrant@citrix.com>; Stefano
> Stabellini <sstabellini@kernel.org>; xen-devel@lists.xenproject.org
> Subject: [PATCH v2 4/4] xen: Avoid VLA
> 
> Avoid using a variable length array.
> 
> We allocate the `dirty_bitmap' buffer only once when we start tracking
> for dirty bits.
> 
> Signed-off-by: Anthony PERARD <anthony.perard@citrix.com>

Reviewed-by: Paul Durrant <paul.durrant@citrix.com>

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Re: [Xen-devel] [Qemu-devel] [PATCH v2 4/4] xen: Avoid VLA
Posted by Philippe Mathieu-Daudé 6 years, 7 months ago 
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@citrix.com>

Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>

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