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SubjectRE: Re: [V4 PATCH 1/2] x86/panic: Replace smp_send_stop() with kdump friendly version in panic path
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Hi Dave,

Thank you for the review.

> From: Dave Young [mailto:dyoung@redhat.com]
> Sent: Friday, August 12, 2016 12:17 PM
>
> Thanks for the update.
> On 08/10/16 at 05:09pm, Hidehiro Kawai wrote:
> > Daniel Walker reported problems which happens when
> > crash_kexec_post_notifiers kernel option is enabled
> > (https://lkml.org/lkml/2015/6/24/44).
> >
> > In that case, smp_send_stop() is called before entering kdump routines
> > which assume other CPUs are still online. As the result, for x86,
> > kdump routines fail to save other CPUs' registers and disable
> > virtualization extensions.
>
> Seems you simplified the changelog, but I think a little more details
> will be helpful to understand the patch. You know sometimes lkml.org
> does not work well.

So, I'll try another archives when I post patch set next time.

> > To fix this problem, call a new kdump friendly function,
> > crash_smp_send_stop(), instead of the smp_send_stop() when
> > crash_kexec_post_notifiers is enabled. crash_smp_send_stop() is a
> > weak function, and it just call smp_send_stop(). Architecture
> > codes should override it so that kdump can work appropriately.
> > This patch only provides x86-specific version.
> >
> > For Xen's PV kernel, just keep the current behavior.
>
> Could you explain a bit about above Xen PV kernel behavior?
>
> BTW, this version looks better, I think I'm fine with this version
> besides of the questions about changelog.

As for Dom0 kernel, it doesn't use crash_kexec routines, and
it relies on panic notifier chain. At the end of the chain,
xen_panic_event is called, and it issues a hypercall which
requests Hypervisor to execute kdump. This means whether
crash_kexec_panic_notifiers is set or not, panic notifiers
are called after smp_send_stop. Even if we save registers
in Dom0 kernel, they seem to be ignored (Hypervisor is responsible
for that). This is why I kept the current behavior for Xen.

For PV DomU kernel, kdump is not supported. For PV HVM
DomU, I'm not sure what will happen on panic because I
couldn't boot PV HVM DomU and test it. But I think it will
work similarly to baremetal kernels with extra cleanups
for Hypervisor.

Best regards,

Hidehiro Kawai

> > Changes in V4:
> > - Keep to use smp_send_stop if crash_kexec_post_notifiers is not set
> > - Rename panic_smp_send_stop to crash_smp_send_stop
> > - Don't change the behavior for Xen's PV kernel
> >
> > Changes in V3:
> > - Revise comments, description, and symbol names
> >
> > Changes in V2:
> > - Replace smp_send_stop() call with crash_kexec version which
> > saves cpu states and cleans up VMX/SVM
> > - Drop a fix for Problem 1 at this moment
> >
> > Reported-by: Daniel Walker <dwalker@fifo99.com>
> > Fixes: f06e5153f4ae (kernel/panic.c: add "crash_kexec_post_notifiers" option)
> > Signed-off-by: Hidehiro Kawai <hidehiro.kawai.ez@hitachi.com>
> > Cc: Dave Young <dyoung@redhat.com>
> > Cc: Baoquan He <bhe@redhat.com>
> > Cc: Vivek Goyal <vgoyal@redhat.com>
> > Cc: Eric Biederman <ebiederm@xmission.com>
> > Cc: Masami Hiramatsu <mhiramat@kernel.org>
> > Cc: Daniel Walker <dwalker@fifo99.com>
> > Cc: Xunlei Pang <xpang@redhat.com>
> > Cc: Thomas Gleixner <tglx@linutronix.de>
> > Cc: Ingo Molnar <mingo@redhat.com>
> > Cc: "H. Peter Anvin" <hpa@zytor.com>
> > Cc: Borislav Petkov <bp@suse.de>
> > Cc: David Vrabel <david.vrabel@citrix.com>
> > Cc: Toshi Kani <toshi.kani@hpe.com>
> > Cc: Andrew Morton <akpm@linux-foundation.org>
> > ---
> > arch/x86/include/asm/kexec.h | 1 +
> > arch/x86/include/asm/smp.h | 1 +
> > arch/x86/kernel/crash.c | 22 +++++++++++++++++---
> > arch/x86/kernel/smp.c | 5 ++++
> > kernel/panic.c | 47 ++++++++++++++++++++++++++++++++++++------
> > 5 files changed, 66 insertions(+), 10 deletions(-)
> >
> > diff --git a/arch/x86/include/asm/kexec.h b/arch/x86/include/asm/kexec.h
> > index d2434c1..282630e 100644
> > --- a/arch/x86/include/asm/kexec.h
> > +++ b/arch/x86/include/asm/kexec.h
> > @@ -210,6 +210,7 @@ struct kexec_entry64_regs {
> >
> > typedef void crash_vmclear_fn(void);
> > extern crash_vmclear_fn __rcu *crash_vmclear_loaded_vmcss;
> > +extern void kdump_nmi_shootdown_cpus(void);
> >
> > #endif /* __ASSEMBLY__ */
> >
> > diff --git a/arch/x86/include/asm/smp.h b/arch/x86/include/asm/smp.h
> > index ebd0c16..f70989c 100644
> > --- a/arch/x86/include/asm/smp.h
> > +++ b/arch/x86/include/asm/smp.h
> > @@ -50,6 +50,7 @@ struct smp_ops {
> > void (*smp_cpus_done)(unsigned max_cpus);
> >
> > void (*stop_other_cpus)(int wait);
> > + void (*crash_stop_other_cpus)(void);
> > void (*smp_send_reschedule)(int cpu);
> >
> > int (*cpu_up)(unsigned cpu, struct task_struct *tidle);
> > diff --git a/arch/x86/kernel/crash.c b/arch/x86/kernel/crash.c
> > index 9616cf7..650830e 100644
> > --- a/arch/x86/kernel/crash.c
> > +++ b/arch/x86/kernel/crash.c
> > @@ -133,15 +133,31 @@ static void kdump_nmi_callback(int cpu, struct pt_regs *regs)
> > disable_local_APIC();
> > }
> >
> > -static void kdump_nmi_shootdown_cpus(void)
> > +void kdump_nmi_shootdown_cpus(void)
> > {
> > nmi_shootdown_cpus(kdump_nmi_callback);
> >
> > disable_local_APIC();
> > }
> >
> > +/* Override the weak function in kernel/panic.c */
> > +void crash_smp_send_stop(void)
> > +{
> > + static int cpus_stopped;
> > +
> > + if (cpus_stopped)
> > + return;
> > +
> > + if (smp_ops.crash_stop_other_cpus)
> > + smp_ops.crash_stop_other_cpus();
> > + else
> > + smp_send_stop();
> > +
> > + cpus_stopped = 1;
> > +}
> > +
> > #else
> > -static void kdump_nmi_shootdown_cpus(void)
> > +void crash_smp_send_stop(void)
> > {
> > /* There are no cpus to shootdown */
> > }
> > @@ -160,7 +176,7 @@ void native_machine_crash_shutdown(struct pt_regs *regs)
> > /* The kernel is broken so disable interrupts */
> > local_irq_disable();
> >
> > - kdump_nmi_shootdown_cpus();
> > + crash_smp_send_stop();
> >
> > /*
> > * VMCLEAR VMCSs loaded on this cpu if needed.
> > diff --git a/arch/x86/kernel/smp.c b/arch/x86/kernel/smp.c
> > index 658777c..68f8cc2 100644
> > --- a/arch/x86/kernel/smp.c
> > +++ b/arch/x86/kernel/smp.c
> > @@ -32,6 +32,8 @@
> > #include <asm/nmi.h>
> > #include <asm/mce.h>
> > #include <asm/trace/irq_vectors.h>
> > +#include <asm/kexec.h>
> > +
> > /*
> > * Some notes on x86 processor bugs affecting SMP operation:
> > *
> > @@ -342,6 +344,9 @@ struct smp_ops smp_ops = {
> > .smp_cpus_done = native_smp_cpus_done,
> >
> > .stop_other_cpus = native_stop_other_cpus,
> > +#if defined(CONFIG_KEXEC_CORE)
> > + .crash_stop_other_cpus = kdump_nmi_shootdown_cpus,
> > +#endif
> > .smp_send_reschedule = native_smp_send_reschedule,
> >
> > .cpu_up = native_cpu_up,
> > diff --git a/kernel/panic.c b/kernel/panic.c
> > index ca8cea1..e6480e2 100644
> > --- a/kernel/panic.c
> > +++ b/kernel/panic.c
> > @@ -71,6 +71,32 @@ void __weak nmi_panic_self_stop(struct pt_regs *regs)
> > panic_smp_self_stop();
> > }
> >
> > +/*
> > + * Stop other CPUs in panic. Architecture dependent code may override this
> > + * with more suitable version. For example, if the architecture supports
> > + * crash dump, it should save registers of each stopped CPU and disable
> > + * per-CPU features such as virtualization extensions.
> > + */
> > +void __weak crash_smp_send_stop(void)
> > +{
> > + static int cpus_stopped;
> > +
> > + /*
> > + * This function can be called twice in panic path, but obviously
> > + * we execute this only once.
> > + */
> > + if (cpus_stopped)
> > + return;
> > +
> > + /*
> > + * Note smp_send_stop is the usual smp shutdown function, which
> > + * unfortunately means it may not be hardened to work in a panic
> > + * situation.
> > + */
> > + smp_send_stop();
> > + cpus_stopped = 1;
> > +}
> > +
> > atomic_t panic_cpu = ATOMIC_INIT(PANIC_CPU_INVALID);
> >
> > /*
> > @@ -164,14 +190,21 @@ void panic(const char *fmt, ...)
> > if (!_crash_kexec_post_notifiers) {
> > printk_nmi_flush_on_panic();
> > __crash_kexec(NULL);
> > - }
> >
> > - /*
> > - * Note smp_send_stop is the usual smp shutdown function, which
> > - * unfortunately means it may not be hardened to work in a panic
> > - * situation.
> > - */
> > - smp_send_stop();
> > + /*
> > + * Note smp_send_stop is the usual smp shutdown function, which
> > + * unfortunately means it may not be hardened to work in a
> > + * panic situation.
> > + */
> > + smp_send_stop();
> > + } else {
> > + /*
> > + * If we want to do crash dump after notifier calls and
> > + * kmsg_dump, we will need architecture dependent extra
> > + * works in addition to stopping other CPUs.
> > + */
> > + crash_smp_send_stop();
> > + }
> >
> > /*
> > * Run any panic handlers, including those that might need to
> >
> >
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