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[PATCH v2 7/9] x86/shadow: reduce effort of hash calculation
- To: "xen-devel@xxxxxxxxxxxxxxxxxxxx" <xen-devel@xxxxxxxxxxxxxxxxxxxx>
- From: Jan Beulich <jbeulich@xxxxxxxx>
- Date: Wed, 11 Jan 2023 14:56:30 +0100
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- Cc: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>, Wei Liu <wl@xxxxxxx>, Roger Pau Monné <roger.pau@xxxxxxxxxx>, Tim Deegan <tim@xxxxxxx>, George Dunlap <george.dunlap@xxxxxxxxxx>
- Delivery-date: Wed, 11 Jan 2023 13:56:41 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
The "n" input is a GFN/MFN value and hence bounded by the physical
address bits in use on a system. The hash quality won't improve by also
including the upper always-zero bits in the calculation. To keep things
as compile-time-constant as they were before, use PADDR_BITS (not
paddr_bits) for loop bounding. This reduces loop iterations from 8 to 5.
While there also drop the unnecessary conversion to an array of unsigned
char, moving the value off the stack altogether (at least with
optimization enabled).
Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
---
I was tempted to also change the type "i" (unsigned) right here, but
then thought this might be going too far ...
---
v2: Also eliminate the unsigned char * alias of "n".
--- a/xen/arch/x86/mm/shadow/common.c
+++ b/xen/arch/x86/mm/shadow/common.c
@@ -1397,10 +1397,13 @@ static unsigned int shadow_get_allocatio
typedef u32 key_t;
static inline key_t sh_hash(unsigned long n, unsigned int t)
{
- unsigned char *p = (unsigned char *)&n;
key_t k = t;
int i;
- for ( i = 0; i < sizeof(n) ; i++ ) k = (u32)p[i] + (k<<6) + (k<<16) - k;
+
+ BUILD_BUG_ON(PADDR_BITS > BITS_PER_LONG + PAGE_SHIFT);
+ for ( i = 0; i < (PADDR_BITS - PAGE_SHIFT + 7) / 8; i++, n >>= 8 )
+ k = (uint8_t)n + (k << 6) + (k << 16) - k;
+
return k % SHADOW_HASH_BUCKETS;
}
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