linux-kernel - Re: [PATCH 04/12] KVM: x86: Replace call-back set_tsc

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Date:	Tue, 6 Oct 2015 19:32:44 +0800
From:	Haozhong Zhang <haozhong.zhang@...el.com>
To:	Paolo Bonzini <pbonzini@...hat.com>
Cc:	Radim Krčmář <rkrcmar@...hat.com>,
	David Matlack <dmatlack@...gle.com>, 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 04/12] KVM: x86: Replace call-back set_tsc_khz() with a
 common function

On Tue, Oct 06, 2015 at 12:40:49PM +0200, Paolo Bonzini wrote:
> 
> 
> 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:
>

This is much better! Thanks Paolo! I'll use this mul_u64_u32_shr() in
the next version.

> 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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