On 28/09/2015 07:38, Haozhong Zhang wrote:
>
> -static u64 __scale_tsc(u64 ratio, u64 tsc)
> -{
> - u64 mult, frac, _tsc;
> -
> - mult = ratio >> 32;
> - frac = ratio & ((1ULL << 32) - 1);
> -
> - _tsc = tsc;
> - _tsc *= mult;
> - _tsc += (tsc >> 32) * frac;
> - _tsc += ((tsc & ((1ULL << 32) - 1)) * frac) >> 32;
> -
> - return _tsc;
> -}
This is basically
return mul_u64_u64_shr(ratio, tsc,
kvm_tsc_scaling_ratio_frac_bits);
except that Linux has no mul_u64_u64_shr function, only mul_u64_u32_shr.
We should implement that function in include/linux/math64.h instead.
For the x86_64 case (or any other CONFIG_ARCH_SUPPORTS_INT128
architecture) we can just write it directly, as is done already for
mul_u64_u32_shr.
For the 32-bit case, here is an implementation of both the
multiplication and the shift, lifted from QEMU:
static inline void mul64(uint64_t *lo, uint64_t *hi,
uint64_t a, uint64_t b)
{
typedef union {
uint64_t ll;
struct {
#ifdef __BIG_ENDIAN
uint32_t high, low;
#else
uint32_t low, high;
#endif
} l;
} LL;
LL rl, rm, rn, rh, a0, b0;
uint64_t c;
a0.ll = a;
b0.ll = b;
rl.ll = (uint64_t)a0.l.low * b0.l.low;
rm.ll = (uint64_t)a0.l.low * b0.l.high;
rn.ll = (uint64_t)a0.l.high * b0.l.low;
rh.ll = (uint64_t)a0.l.high * b0.l.high;
c = (uint64_t)rl.l.high + rm.l.low + rn.l.low;
rl.l.high = c;
c >>= 32;
c = c + rm.l.high + rn.l.high + rh.l.low;
rh.l.low = c;
rh.l.high += (uint32_t)(c >> 32);
*lo = rl.ll;
*hi = rh.ll;
}
static inline void rshift128(uint64_t *lo, uint64_t *hi, int n)
{
uint64_t h;
if (!n) {
return;
}
h = *hi >> (n & 63);
if (n >= 64) {
*hi = 0;
*lo = h;
} else {
*lo = (*lo >> n) | (*hi << (64 - n));
*hi = h;
}
}
and you can easily reuse this code in Linux with just uintNN_t types
changed to uNN + some extra cleanups when it's placed in a single functions.
Paolo
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