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[PATCH 08/11] x86/shadow: reduce effort of hash calculation
- To: "xen-devel@xxxxxxxxxxxxxxxxxxxx" <xen-devel@xxxxxxxxxxxxxxxxxxxx>
- From: Jan Beulich <jbeulich@xxxxxxxx>
- Date: Thu, 5 Jan 2023 17:05:59 +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: Thu, 05 Jan 2023 16:06:06 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
The "n" input is a GFN 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 cast to u32.
Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
---
I was tempted to also change the types of "p" (pointer to const) and "i"
(unsigned) right here (and perhaps even the "byte" in the comment ahead
of the function), but then thought this might be going too far ...
--- a/xen/arch/x86/mm/shadow/common.c
+++ b/xen/arch/x86/mm/shadow/common.c
@@ -1400,7 +1400,11 @@ static inline key_t sh_hash(unsigned lon
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++ )
+ k = p[i] + (k << 6) + (k << 16) - k;
+
return k % SHADOW_HASH_BUCKETS;
}
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