[xen master] x86/shadow: reduce effort of hash calculation

[patchbot@xxxxxxx]

commit d2123363788cc60b1d33123d93d27a49e5a37a43
Author:     Jan Beulich <jbeulich@xxxxxxxx>
AuthorDate: Thu Jan 12 11:12:35 2023 +0100
Commit:     Jan Beulich <jbeulich@xxxxxxxx>
CommitDate: Thu Jan 12 11:12:35 2023 +0100

    x86/shadow: reduce effort of hash calculation
    
    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>
    Reviewed-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
---
 xen/arch/x86/mm/shadow/common.c | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/xen/arch/x86/mm/shadow/common.c b/xen/arch/x86/mm/shadow/common.c
index 3f2caf1598..0c405ea5f3 100644
--- 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_allocation(struct domain 
*d)
 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;
 }
 
--
generated by git-patchbot for /home/xen/git/xen.git#master