On 06/10/2015 06:06, Haozhong Zhang wrote:
> Alternatively, it's also possible to follow David's comment to use
> divq on x86_64 to keep both precision and safety. On i386, it just
> falls back to above truncating approach.
khz is just 32 bits, so we can do a 96/32 division. And because this is
a slow path, we can code a generic u64*u32/u32 function and use it to do
(1 << kvm_tsc_scaling_ratio_frac_bits) * khz / tsc_khz:
diff --git a/include/linux/math64.h b/include/linux/math64.h
index c45c089bfdac..5b70af4fa386 100644
--- a/include/linux/math64.h
+++ b/include/linux/math64.h
@@ -142,6 +142,13 @@ static inline u64 mul_u64_u32_shr(u64 a, u32 mul,
unsigned int shift)
}
#endif /* mul_u64_u32_shr */
+#ifndef mul_u64_u32_div
+static inline u64 mul_u64_u32_div(u64 x, u32 num, u32 den)
+{
+ return (u64)(((unsigned __int128)a * mul) / den);
+}
+#endif
+
#else
#ifndef mul_u64_u32_shr
@@ -161,6 +168,35 @@ static inline u64 mul_u64_u32_shr(u64 a, u32 mul,
unsigned int shift)
}
#endif /* mul_u64_u32_shr */
+#ifndef mul_u64_u32_div
+static inline u64 mul_u64_u32_div(u64 a, u32 num, u32 den)
+{
+ union {
+ u64 ll;
+ struct {
+#ifdef __BIG_ENDIAN
+ u32 high, low;
+#else
+ u32 low, high;
+#endif
+ } l;
+ } u, rl, rh;
+
+ u.ll = a;
+ rl.ll = (u64)u.l.low * num;
+ rh.ll = (u64)u.l.high * num + rl.l.high;
+
+ /* Bits 32-63 of the result will be in rh.l.low. */
+ rl.l.high = do_div(rh.ll, den);
+
+ /* Bits 0-31 of the result will be in rl.l.low. */
+ do_div(rl.ll, den);
+
+ rl.l.high = rh.l.low;
+ return rl.ll;
+}
+#endif
+
#endif
#endif /* _LINUX_MATH64_H */
--
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