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SubjectRe: new text patching for review
* Zachary Amsden (zach@vmware.com) wrote:
> Mathieu Desnoyers wrote:
> >Yes, kprobes is case 1: atomic update. And we don't even have to bother
> >about Intel's erratum. This one is ok. That's mainly the
> >alternatives/paravirt code I worry about.
> >
>
> Paravirt and alternatives should all be ok because they are done before
> SMP bringup and with NMIs disabled. NMI watchdog is not setup until
> smp_prepare_cpus/check_nmi_watchdog, which happens way later, not during
> parse_args/setup_nmi_watchdog, which just decides which type of watchdog
> to setup.
>

I'm not so sure about this. You are right in that it has nothing to do
with parse_args, but I just went in detail through the source, and the
order seems to be:

1 - NMI is activated
2 - MCE is activated
3 - alternatives are applied

In detail:

start_kernel()
...
include/asm-i386/smp.h:smp_prepare_boot_cpu()
smp_ops.smp_prepare_boot_cpu()
arch/i386/kernel/smpboot.c:native_smp_prepare_boot_cpu()
arch/i386/kernel/smpboot.c:native_smp_prepare_cpus()
arch/i386/kernel/smpboot.c:smp_boot_cpus()
arch/i386/kernel/apic.c:setup_local_APIC()
arch/i386/kernel/nmi.c:setup_apic_nmi_watchdog()
arch/i386/kernel/cpu/perfctr-watchdog.c:lapic_watchdog_init()
wd_ops->setup() (e.g. setup_intel_arch_watchdog)
...
arch/i386/kernel/cpu/bugs.c:check_bugs()
arch/i386/kernel/cpu/common.c:identify_boot_cpu()
identify_cpu()
arch/i386/kernel/cpu/mcheck/mce.c:mcheck_init()
arch/i386/kernel/cpu/mcheck/p4.c:intel_p4_mcheck_init()
...
arch/i386/kernel/alternative_instructions()
...

Therefore, applying alternatives instructions seems to be done after NMI
and MCE init at boot time, or am I misunderstanding something ?


> I originally considered the NMI problem for paravirt-ops patching done
> during module load, and found that I would need to modify
> stop_machine_run to have an architecture specific callout to mask and
> unmask NMIs. I didn't imagine that would be very popular, and VMI was
> the only paravirt-ops that were considering module load time patching,
> so I flushed it.
>
> You get some other nasty issues as well with run-time switching, like
> missing early init calls (in particular, we would have to go to some
> heroics to retake the land surrounding the APIC local timer interrupt).
>
> Zach

--
Mathieu Desnoyers
Computer Engineering Ph.D. Student, Ecole Polytechnique de Montreal
OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F BA06 3F25 A8FE 3BAE 9A68
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