Re: [PATCH 04/12] KVM: x86: Replace call-back set_tsc_khz() with a common function

[Paolo Bonzini <pbonzini@redhat.com>]

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 */