linux-kernel - Re: [PATCH] KVM: tsc deadline timer works only when hrtimer high resolution configured

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Date:	Thu, 06 Sep 2012 16:58:32 +0300
From:	Avi Kivity <avi@...hat.com>
To:	"Liu, Jinsong" <jinsong.liu@...el.com>
CC:	Marcelo Tosatti <mtosatti@...hat.com>, kvm <kvm@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] KVM: tsc deadline timer works only when hrtimer high
 resolution configured

On 09/04/2012 06:07 PM, Liu, Jinsong wrote:
> From 728a17e2de591b557c3c8ba31076b4bf2ca5ab42 Mon Sep 17 00:00:00 2001
> From: Liu, Jinsong <jinsong.liu@...el.com>
> Date: Wed, 5 Sep 2012 03:18:15 +0800
> Subject: [PATCH] KVM: tsc deadline timer works only when hrtimer high resolution configured
> 
> This is for 2 reasons:
> 1. it's pointless to enable tsc deadline timer to guest when kernel hrtimer
> not configured as high resolution, since that would be un-precise based on wheel;
> 2. tsc deadline timer based on hrtimer, setting a leftmost node to rb tree
> and then do hrtimer reprogram. If hrtimer not configured as high resolution,
> hrtimer_enqueue_reprogram do nothing and would make tsc deadline timer fail.
> 
> 
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 148ed66..0e64997 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -2210,7 +2210,11 @@ int kvm_dev_ioctl_check_extension(long ext)
>  		r = kvm_has_tsc_control;
>  		break;
>  	case KVM_CAP_TSC_DEADLINE_TIMER:
> +#ifdef CONFIG_HIGH_RES_TIMERS
>  		r = boot_cpu_has(X86_FEATURE_TSC_DEADLINE_TIMER);
> +#else
> +		r = 0;
> +#endif
>  		break;

I prefer a patch making kvm for x86 depend on hrtimers.  kvm already
provides a high resolution timer to the guest in the local apic, backing
it with the jiffies event source will likely cause some guests to
malfunction.


-- 
error compiling committee.c: too many arguments to function
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