linux-kernel - Re: [PATCH 15/17] Fix AMD C1 TSC desynchronization

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Date:	Tue, 15 Jun 2010 11:47:20 +0300
From:	Avi Kivity <avi@...hat.com>
To:	Zachary Amsden <zamsden@...hat.com>
CC:	mtosatti@...hat.com, glommer@...hat.com, kvm@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 15/17] Fix AMD C1 TSC desynchronization

On 06/15/2010 10:34 AM, Zachary Amsden wrote:
> Some AMD based machines can have TSC drift when in C1 HLT state because
> despite attempting to scale the TSC increment when dividing down the
> P-state, the processor may return to full P-state to service cache
> probes.  The TSC of halted CPUs can advance faster than that of running
> CPUs as a result, causing unpredictable TSC drift.
>
> We implement a recommended workaround, which is disabling C1 clock ramping.
> ---
>   arch/x86/kvm/x86.c |   45 +++++++++++++++++++++++++++++++++++++++++++++
>   1 files changed, 45 insertions(+), 0 deletions(-)
>
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index ef847ee..8e836e9 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -56,6 +56,11 @@
>   #include<asm/i387.h>
>   #include<asm/xcr.h>
>
> +#ifdef CONFIG_K8_NB
> +#include<linux/pci.h>
> +#include<asm/k8.h>
> +#endif
> +
>   #define MAX_IO_MSRS 256
>   #define CR0_RESERVED_BITS						\
>   	(~(unsigned long)(X86_CR0_PE | X86_CR0_MP | X86_CR0_EM | X86_CR0_TS \
> @@ -4287,10 +4292,43 @@ static struct notifier_block kvmclock_cpu_notifier_block = {
>   	.priority = -INT_MAX
>   };
>
> +static u8 disabled_c1_ramp = 0;
> +
>   static void kvm_timer_init(void)
>   {
>   	int cpu;
>
> +	/*
> +	 * AMD processors can de-synchronize TSC on halt in C1 state, because
> +	 * processors in lower P state will have TSC scaled properly during
> +	 * normal operation, but will have TSC scaled improperly while
> +	 * servicing cache probes.  Because there is no way to determine how
> +	 * TSC was adjusted during cache probes, there are two solutions:
> +	 * resynchronize after halt, or disable C1-clock ramping.
> +	 *
> +	 * We implemenent solution 2.
> +	 */
> +#ifdef CONFIG_K8_NB
> +	if (boot_cpu_data.x86_vendor == X86_VENDOR_AMD&&
> +	    boot_cpu_data.x86 == 0x0f&&
> +	    !boot_cpu_has(X86_FEATURE_CONSTANT_TSC)) {
> +		struct pci_dev *nb;
> +		int i;
> +		cache_k8_northbridges();
> +		for (i = 0; i<  num_k8_northbridges; i++) {
> +			u8 byte;
> +			nb = k8_northbridges[i];
> +			pci_read_config_byte(nb, 0x87,&byte);
> +			if (byte&  1) {
> +				printk(KERN_INFO "%s: AMD C1 clock ramping detected, performing workaround\n", __func__);
> +				disabled_c1_ramp = byte;
> +				pci_write_config_byte(nb, 0x87, byte&  0xFC);
> +
> +			}
> +		}
> +	}
> +#endif
> +
>   	register_hotcpu_notifier(&kvmclock_cpu_notifier_block);
>   	if (!boot_cpu_has(X86_FEATURE_CONSTANT_TSC)) {
>   		cpufreq_register_notifier(&kvmclock_cpufreq_notifier_block,
> @@ -4402,6 +4440,13 @@ void kvm_arch_exit(void)
>   	unregister_hotcpu_notifier(&kvmclock_cpu_notifier_block);
>   	kvm_x86_ops = NULL;
>   	kvm_mmu_module_exit();
> +#ifdef CONFIG_K8_NB
> +	if (disabled_c1_ramp) {
> +		struct pci_dev **nb;
> +		for (nb = k8_northbridges; *nb; nb++)
> +			pci_write_config_byte(*nb, 0x87, disabled_c1_ramp);
> +	}
> +#endif
>   }
>    

Such platform hackery should be in the platform code, not in kvm.  kvm 
might request to enable it (why not enable it unconditionally?  should 
we disable it on hardware_disable()?


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

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