Re: [Qemu-devel] [PATCH] kvm: clear guest TSC on reset

["Fernando Luis Vázquez Cao" <fernando_b1@lab.ntt.co.jp>]

I realized that the TSC reset should be done in QEMU
so I will be replying with a QEMU patch instead of a
KVM one.

- Fernando


On 12/03/2013 05:04 PM, Fernando Luis Vázquez Cao wrote:
> I think there is a problem with the current patch, so please
> ignore for the moment. I will be replying with an update ASAP.
>
> Sorry for the noise.
>
> - Fernando
>
>
> On 12/03/2013 04:08 PM, Fernando Luis Vázquez Cao wrote:
>> VCPU TSC is not cleared by a warm reset (*), which leaves many Linux
>> guests vulnerable to the overflow in cyc2ns_offset fixed by upstream
>> commit 9993bc635d01a6ee7f6b833b4ee65ce7c06350b1 ("sched/x86: Fix 
>> overflow
>> in cyc2ns_offset").
>>
>> To put it in a nutshell, if a Linux guest without the patch above 
>> applied
>> has been up more than 208 days and attempts a warm reset chances are 
>> that
>> the newly booted kernel will panic or hang.
>>
>> (*) Intel Xeon E5 processors show the same broken behavior due to
>>      the errata "TSC is Not Affected by Warm Reset" (Intel® Xeon®
>>      Processor E5 Family Specification Update - August 2013): "The
>>      TSC (Time Stamp Counter MSR 10H) should be cleared on
>>      reset. Due to this erratum the TSC is not affected by warm
>>      reset."
>>
>> Cc: stable@vger.kernel.org
>> Cc: Will Auld <will.auld@intel.com>
>> Signed-off-by: Fernando Luis Vazquez Cao <fernando@oss.ntt.co.jp>
>> ---
>>
>> diff -urNp linux-3.13-rc2-orig/arch/x86/kvm/x86.c 
>> linux-3.13-rc2/arch/x86/kvm/x86.c
>> --- linux-3.13-rc2-orig/arch/x86/kvm/x86.c    2013-11-30 
>> 05:57:14.000000000 +0900
>> +++ linux-3.13-rc2/arch/x86/kvm/x86.c    2013-12-03 
>> 14:51:53.747600839 +0900
>> @@ -6716,18 +6716,24 @@ int kvm_arch_vcpu_setup(struct kvm_vcpu
>>       return r;
>>   }
>>   -int kvm_arch_vcpu_postcreate(struct kvm_vcpu *vcpu)
>> +static void kvm_tsc_reset(struct kvm_vcpu *vcpu)
>>   {
>> -    int r;
>>       struct msr_data msr;
>>   -    r = vcpu_load(vcpu);
>> -    if (r)
>> -        return r;
>>       msr.data = 0x0;
>>       msr.index = MSR_IA32_TSC;
>>       msr.host_initiated = true;
>>       kvm_write_tsc(vcpu, &msr);
>> +}
>> +
>> +int kvm_arch_vcpu_postcreate(struct kvm_vcpu *vcpu)
>> +{
>> +    int r;
>> +
>> +    r = vcpu_load(vcpu);
>> +    if (r)
>> +        return r;
>> +    kvm_tsc_reset(vcpu);
>>       vcpu_put(vcpu);
>>         return r;
>> @@ -6770,6 +6776,10 @@ void kvm_vcpu_reset(struct kvm_vcpu *vcp
>>         kvm_pmu_reset(vcpu);
>>   +    kvm_tsc_reset(vcpu);
>> +    if (guest_cpuid_has_tsc_adjust(vcpu))
>> +        vcpu->arch.ia32_tsc_adjust_msr = 0x0;
>> +
>>       memset(vcpu->arch.regs, 0, sizeof(vcpu->arch.regs));
>>       vcpu->arch.regs_avail = ~0;
>>       vcpu->arch.regs_dirty = ~0;
>>
>>
>