linux-kernel - Re: [PATCH 03/12] KVM: x86: Add a common TSC scaling function

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Date:	Tue, 29 Sep 2015 09:51:53 +0800
From:	Haozhong Zhang <haozhong.zhang@...el.com>
To:	Paolo Bonzini <pbonzini@...hat.com>
Cc:	kvm@...r.kernel.org, Gleb Natapov <gleb@...nel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>, x86@...nel.org,
	Joerg Roedel <joro@...tes.org>,
	Wanpeng Li <wanpeng.li@...ux.intel.com>,
	Xiao Guangrong <guangrong.xiao@...ux.intel.com>,
	Mihai Donțu <mdontu@...defender.com>,
	Andy Lutomirski <luto@...nel.org>,
	Kai Huang <kai.huang@...ux.intel.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 03/12] KVM: x86: Add a common TSC scaling function

On Mon, Sep 28, 2015 at 10:12:37PM +0200, Paolo Bonzini wrote:
> 
> 
> 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

Thanks! I'll add mul_u64_u64_shr() and replace __scale_tsc().

- Haozhong
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