Here is the revised commit description reflecting Dave's
comment.  Cc list was copied from -mm version.

From: Hidehiro Kawai <hidehiro.kawai...@hitachi.com>
Subject: x86/panic: replace smp_send_stop() with kdump friendly version in 
panic path

This patch fixes a problem reported by Daniel Walker
(https://lkml.org/lkml/2015/6/24/44).

When kernel panics with crash_kexec_post_notifiers kernel parameter
enabled, other CPUs are stopped by smp_send_stop() instead of
machine_crash_shutdown() in __crash_kexec() path.

  panic()
    if crash_kexec_post_notifiers == 1
      smp_send_stop()
      atomic_notifier_call_chain()
      kmsg_dump()
    __crash_kexec()
      machine_crash_shutdown()

Different from smp_send_stop(), machine_crash_shutdown() stops other
CPUs with extra works for kdump.  So, if smp_send_stop() stops other
CPUs in advance, these extra works won't be done.  For x86, kdump
routines miss to save other CPUs' registers and disable virtualization
extensions.

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.
As for Dom0, it doesn't use crash_kexec routines, and it relies on
panic notifier chain.  At the end of the chain, a hypercall is
issued which requests the hypervisor to execute kdump.  This means
regardless of crash_kexec_post_notifiers setting, smp_send_stop().
For PV HVM, it would work similarly to baremetal kernels with extra
cleanups for hypervisor.  It doesn't need additional care.

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

Fixes: f06e5153f4ae (kernel/panic.c: add "crash_kexec_post_notifiers" option)
Link: 
http://lkml.kernel.org/r/20160810080948.11028.15344.st...@sysi4-13.yrl.intra.hitachi.co.jp
Signed-off-by: Hidehiro Kawai <hidehiro.kawai...@hitachi.com>
Reported-by: Daniel Walker <dwal...@fifo99.com>
Cc: Dave Young <dyo...@redhat.com>
Cc: Baoquan He <b...@redhat.com>
Cc: Vivek Goyal <vgo...@redhat.com>
Cc: Eric Biederman <ebied...@xmission.com>
Cc: Masami Hiramatsu <mhira...@kernel.org>
Cc: Daniel Walker <dwal...@fifo99.com>
Cc: Xunlei Pang <xp...@redhat.com>
Cc: Thomas Gleixner <t...@linutronix.de>
Cc: Ingo Molnar <mi...@redhat.com>
Cc: "H. Peter Anvin" <h...@zytor.com>
Cc: Borislav Petkov <b...@suse.de>
Cc: David Vrabel <david.vra...@citrix.com>
Cc: Toshi Kani <toshi.k...@hpe.com>
Cc: Ralf Baechle <r...@linux-mips.org>
Cc: David Daney <david.da...@cavium.com>
Cc: Aaro Koskinen <aaro.koski...@iki.fi>
Cc: "Steven J. Hill" <steven.h...@cavium.com>
Cc: Corey Minyard <cminy...@mvista.com>
Signed-off-by: Andrew Morton <a...@linux-foundation.org>

> -----Original Message-----
> From: Hidehiro Kawai
> Hi Xunlei,
> 
> > From: Xunlei Pang [mailto:xp...@redhat.com]
> > Sent: Tuesday, September 20, 2016 4:40 PM
> > On 2016/08/15/ at 19:22, Hidehiro Kawai wrote:
> > > Hi Dave,
> > >
> > > Thank you for the review.
> > >
> > >> From: Dave Young [mailto:dyo...@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.
> >
> > Hi Hidehiro Kawai,
> >
> > What's the status of this patch set, are you going to send an updated 
> > version?
> 
> Sorry, I misunderstood what Dave said, and I thought I don't
> need to revise the patch set.
> 
> Currently, this patch set is in -mm, then -next tree.
> What I need to fix is only commit descriptions, so I'm going to
> post revised descriptions as replies to this thread, and then
> request to Andrew to replace them if there is no objection.
> (or should I resend the whole patch set?)
> 
> Regards,
> Hidehiro Kawai
> 
> > >>> 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 <dwal...@fifo99.com>
> > >>> Fixes: f06e5153f4ae (kernel/panic.c: add "crash_kexec_post_notifiers" 
> > >>> option)
> > >>> Signed-off-by: Hidehiro Kawai <hidehiro.kawai...@hitachi.com>
> > >>> Cc: Dave Young <dyo...@redhat.com>
> > >>> Cc: Baoquan He <b...@redhat.com>
> > >>> Cc: Vivek Goyal <vgo...@redhat.com>
> > >>> Cc: Eric Biederman <ebied...@xmission.com>
> > >>> Cc: Masami Hiramatsu <mhira...@kernel.org>
> > >>> Cc: Daniel Walker <dwal...@fifo99.com>
> > >>> Cc: Xunlei Pang <xp...@redhat.com>
> > >>> Cc: Thomas Gleixner <t...@linutronix.de>
> > >>> Cc: Ingo Molnar <mi...@redhat.com>
> > >>> Cc: "H. Peter Anvin" <h...@zytor.com>
> > >>> Cc: Borislav Petkov <b...@suse.de>
> > >>> Cc: David Vrabel <david.vra...@citrix.com>
> > >>> Cc: Toshi Kani <toshi.k...@hpe.com>
> > >>> Cc: Andrew Morton <a...@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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