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

Zachary Amsden
2010-06-15T09:21:18Z

On 06/14/2010 10:47 PM, Avi Kivity wrote:
> 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()?

I actually have some negative effects to report from this patch - when
under stress, my laptop spontaneously shut down. Thermal problems?

I agree it is complete hackery. I do not recommend this patch for
upstream inclusion unless it is proposed also by someone more familiar
with the hardware.

However, it was required for my testing.