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

[Fernando Luis Vázquez Cao]

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: address@hidden
> > Cc: Will Auld <address@hidden>
> > Signed-off-by: Fernando Luis Vazquez Cao <address@hidden>
> > ---
> >
> > 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;