From: Markus Armbruster <arm...@redhat.com>
The function's stated contract is simple enough: "round down to the
nearest power of 2". Suggests the domain is the representable numbers
>= 1, because that's the smallest power of two.
The implementation doesn't check for domain errors, but returns
garbage instead:
* For negative arguments, pow2floor() returns -2^63, which is not even
a power of two, let alone the nearest one.
What sort of works is passing *unsigned* arguments >= 2^63. The
implicit conversion to signed is implementation defined, but
commonly yields the (negative) two's complement. pow2floor() then
returns -2^63. Callers that convert that back to unsigned get the
correct value 2^63.
* For a zero argument, pow2floor() shifts right by 64. Undefined
behavior. Common actual behavior is to shift by 0, yielding -2^63.
Fix by switching from int64_t to uint64_t and amending the contract to
map zero to zero.
Callers are fine with that:
* memory_access_size()
This function makes no sense unless the argument is positive and the
return value fits into int.
* raw_refresh_limits()
Passes an int between 1 and BDRV_REQUEST_MAX_BYTES.
* iscsi_refresh_limits()
Passes an integer between 0 and INT_MAX, converts the result to
uint32_t. Passing zero would be undefined behavior, but commonly
yield zero. The patch gives us the zero without the undefined
behavior.
* cache_init()
Passes a positive int64_t argument.
* xbzrle_cache_resize()
Passes a positive int64_t argument (>= TARGET_PAGE_SIZE, actually).
* spapr_node0_size()
Passes a positive uint64_t argument, and converts the result to
hwaddr, i.e. uint64_t.
* spapr_populate_memory()
Passes a positive hwaddr argument, and converts the result to
hwaddr.
Cc: Juan Quintela <quint...@redhat.com>
Cc: Dr. David Alan Gilbert <dgilb...@redhat.com>
Cc: Eric Blake <ebl...@redhat.com>
Cc: Peter Maydell <peter.mayd...@linaro.org>
Cc: Alexey Kardashevskiy <a...@ozlabs.ru>
Signed-off-by: Markus Armbruster <arm...@redhat.com>
Message-Id: <1501148776-16890-3-git-send-email-arm...@redhat.com>
Reviewed-by: Eric Blake <ebl...@redhat.com>
Signed-off-by: Dr. David Alan Gilbert <dgilb...@redhat.com>
---
include/qemu/host-utils.h | 13 ++++++++-----
1 file changed, 8 insertions(+), 5 deletions(-)
diff --git a/include/qemu/host-utils.h b/include/qemu/host-utils.h
index 95cf4f4163..6c6005f5cf 100644
--- a/include/qemu/host-utils.h
+++ b/include/qemu/host-utils.h
@@ -369,13 +369,16 @@ static inline bool is_power_of_2(uint64_t value)
return !(value & (value - 1));
}
-/* round down to the nearest power of 2*/
-static inline int64_t pow2floor(int64_t value)
+/**
+ * Return @value rounded down to the nearest power of two or zero.
+ */
+static inline uint64_t pow2floor(uint64_t value)
{
- if (!is_power_of_2(value)) {
- value = 0x8000000000000000ULL >> clz64(value);
+ if (!value) {
+ /* Avoid undefined shift by 64 */
+ return 0;
}
- return value;
+ return 0x8000000000000000ull >> clz64(value);
}
/* round up to the nearest power of 2 (0 if overflow) */
--
2.13.5
