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

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Date:	Mon, 5 Oct 2015 13:46:25 -0700
From:	David Matlack <dmatlack@...gle.com>
To:	Radim Krčmář <rkrcmar@...hat.com>
Cc:	Haozhong Zhang <haozhong.zhang@...el.com>,
	kvm list <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" <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 5, 2015 at 12:53 PM, Radim Krčmář <rkrcmar@...hat.com> 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.
>
>> +     /* 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.

We can easily avoid losing precision on x86_64 (divq allows a 128-bit
dividend). 32-bit can just lose the 16 bits of precision (TSC scaling
is only available on SkyLake, and I'd be surprised if there were
many hosts running KVM in protected mode on SkyLake :)).

>
> Please mention that we are truncating on purpose :)
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