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

Avi Kivity
2010-06-15T08:47:25Z

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