Re: [PATCH] kvm: clear guest TSC on reset

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

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;
>
>