Re: [Qemu-devel] [PATCH] target-i386: clear guest TSC on reset

[Fernando Luis Vazquez Cao]

(2013/12/05 18:28), Paolo Bonzini wrote:
> Il 05/12/2013 07:15, Fernando Luis Vázquez Cao ha scritto:
> > 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>
> > Cc: Marcelo Tosatti <address@hidden>
> > Signed-off-by: Fernando Luis Vazquez Cao <address@hidden>
> I agree that the bug is in QEMU.  One small nit in your patch is that
> you should reset env->tsc_adjust and env->tsc in x86_cpu_reset.  This
> would already be pretty good.

Yes, that is certainly cleaner (I should try not to take shortcuts...). 
I am attaching
an updated patch (I apologize for not sending it inline - for reasons 
better left
untold I am writing this on a problematic email client :) ).



> However, a bigger problem is that env->tsc is a useless duplicate of
> "cpu_get_ticks() + env->tsc_adjust".  It would be nice to drop env->tsc
> completely except for migration backwards compatibility.  Thus you can:
>
> - fill in env->tsc as mentioned above from target-i386/machine.c's
> cpu_pre_save function.  This guarantees backwards compatibility.
>
> - add a function cpu_set_ticks(int64_t ticks) to cpus.c.  The function
> does nothing if use_icount is true, otherwise it needs to have (roughly)
> the opposite logic compared to cpu_get_ticks.  You then call this
> function from x86_cpu_reset instead of setting env->tsc.  You can
> similarly call this function from kvm_get_msrs.
>
> - add a function kvm_set_ticks(int64_t ticks) to kvm-all.c and
> kvm-stub.c.  For kvm-all.c it calls kvm_arch_set_ticks(CPUState *cpu,
> int64_t ticks) in target-*/kvm.c.  The kvm_arch_set_tsc() function has a
> dummy implementation for all architectures except x86.  For x86 it calls
> KVM_SET_MSRS passing "ticks + env->tsc_offset".
>
> - call kvm_set_ticks() from cpu_set_ticks() and cpu_enable_ticks()
>
> Can you do this?

Can you pick my original fix first? I can do what you suggest in a follow-up
patch.

Thanks,
Fernando

target-i386-clear-guest-TSC-on-reset-v2.patch
Description: Text document