[PATCH for-4.19] xen/xmalloc: XMEM_POOL_POISON improvements

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

When in use, the spew:

  (XEN) Assertion '!memchr_inv(b->ptr.buffer + MIN_BLOCK_SIZE, POISON_BYTE, 
(b->size & BLOCK_SIZE_MASK) - MIN_BLOCK_SIZE)' failed at 
common/xmalloc_tlsf.c:246

is unweidly and meaningless to non-Xen developers.  Therefore:

 * Switch to IS_ENABLED().  There's no need for full #ifdef-ary.
 * Pull memchr_inv() out into the if(), and provide a better error message.

Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
---
CC: George Dunlap <George.Dunlap@xxxxxxxxxxxxx>
CC: Jan Beulich <JBeulich@xxxxxxxx>
CC: Stefano Stabellini <sstabellini@xxxxxxxxxx>
CC: Wei Liu <wl@xxxxxxx>
CC: Julien Grall <julien@xxxxxxx>
CC: Roger Pau Monné <roger.pau@xxxxxxxxxx>

Observations from the debugging of:
  https://github.com/Dasharo/dasharo-issues/issues/488

There's a further bug.  XMEM_POOL_POISON can be enabled in release builds,
where the ASSERT() gets dropped.

However, upping to a BUG() can't provide any helpful message out to the user.

I tried modifying BUG() to take an optional message, but xen/bug.h needs
untangling substantially before that will work, and I don't have time right now.
---
 xen/common/xmalloc_tlsf.c | 15 +++++++--------
 1 file changed, 7 insertions(+), 8 deletions(-)

diff --git a/xen/common/xmalloc_tlsf.c b/xen/common/xmalloc_tlsf.c
index 349b31cb4cc1..13305cd87c2f 100644
--- a/xen/common/xmalloc_tlsf.c
+++ b/xen/common/xmalloc_tlsf.c
@@ -249,11 +249,11 @@ static inline void EXTRACT_BLOCK(struct bhdr *b, struct 
xmem_pool *p, int fl,
     }
     b->ptr.free_ptr = (struct free_ptr) {NULL, NULL};
 
-#ifdef CONFIG_XMEM_POOL_POISON
-    if ( (b->size & BLOCK_SIZE_MASK) > MIN_BLOCK_SIZE )
-        ASSERT(!memchr_inv(b->ptr.buffer + MIN_BLOCK_SIZE, POISON_BYTE,
-                           (b->size & BLOCK_SIZE_MASK) - MIN_BLOCK_SIZE));
-#endif /* CONFIG_XMEM_POOL_POISON */
+    if ( IS_ENABLED(CONFIG_XMEM_POOL_POISON) &&
+         (b->size & BLOCK_SIZE_MASK) > MIN_BLOCK_SIZE &&
+         memchr_inv(b->ptr.buffer + MIN_BLOCK_SIZE, POISON_BYTE,
+                    (b->size & BLOCK_SIZE_MASK) - MIN_BLOCK_SIZE) )
+        ASSERT(!"XMEM Pool corruption found");
 }
 
 /**
@@ -261,11 +261,10 @@ static inline void EXTRACT_BLOCK(struct bhdr *b, struct 
xmem_pool *p, int fl,
  */
 static inline void INSERT_BLOCK(struct bhdr *b, struct xmem_pool *p, int fl, 
int sl)
 {
-#ifdef CONFIG_XMEM_POOL_POISON
-    if ( (b->size & BLOCK_SIZE_MASK) > MIN_BLOCK_SIZE )
+    if ( IS_ENABLED(CONFIG_XMEM_POOL_POISON) &&
+         (b->size & BLOCK_SIZE_MASK) > MIN_BLOCK_SIZE )
         memset(b->ptr.buffer + MIN_BLOCK_SIZE, POISON_BYTE,
                (b->size & BLOCK_SIZE_MASK) - MIN_BLOCK_SIZE);
-#endif /* CONFIG_XMEM_POOL_POISON */
 
     b->ptr.free_ptr = (struct free_ptr) {NULL, p->matrix[fl][sl]};
     if ( p->matrix[fl][sl] )
-- 
2.30.2