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Date: Tue, 6 Oct 2015 12:06:47 +0800 From: Haozhong Zhang <haozhong.zhang@...el.com> To: Radim Krčmář <rkrcmar@...hat.com> Cc: David Matlack <dmatlack@...gle.com>, kvm@...r.kernel.org, Gleb Natapov <gleb@...nel.org>, Paolo Bonzini <pbonzini@...hat.com>, 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 Mon, Oct 05, 2015 at 09:53:26PM +0200, Radim Krčmář wrote: > 2015-09-28 13:38+0800, Haozhong Zhang: > > Both VMX and SVM propagate virtual_tsc_khz in the same way, so this > > patch removes the call-back set_tsc_khz() and replaces it with a common > > function. > > > > Signed-off-by: Haozhong Zhang <haozhong.zhang@...el.com> > > --- > > diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c > > +static void set_tsc_khz(struct kvm_vcpu *vcpu, u32 user_tsc_khz, bool scale) > > +{ > > + u64 ratio, khz; > | [...] > > + khz = user_tsc_khz; > > I'd use "user_tsc_khz" directly. > I'll do so. > > + /* TSC scaling required - calculate ratio */ > > + shift = (kvm_tsc_scaling_ratio_frac_bits <= 32) ? > > + kvm_tsc_scaling_ratio_frac_bits : 32; > > + ratio = khz << shift; > > + do_div(ratio, tsc_khz); > > + ratio <<= (kvm_tsc_scaling_ratio_frac_bits - shift); > > VMX is losing 16 bits by this operation; normal fixed point division > could get us a smaller drift (and an one-liner here) ... > at 4.3 GHz, 32 instead of 48 bits after decimal point translate to one > "lost" TSC tick per second, in the worst case. > > Please mention that we are truncating on purpose :) It's intentional to avoid the potential overflow in khz << kvm_tsc_scaling_ratio_frac_bits. For VMX where kvm_tsc_scaling_ratio_frac_bits == 48, the above expression is only safe to left shift a pretty small khz (< 2^16 KHz or 65.5 MHz). Thus, I decided to sacrifice the precision for safety. I chose to truncate at the boundary of 32 bits which can handle khz as large as 4294 GHz. Though this truncation results in losing TSC ticks when khz is larger than 4.3 GHz, the lost is however pretty small compared with the large khz. 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. - Haozhong -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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