Re: [PATCH 0/4] x86/Hyper-V: Panic code path fixes

[Tianyu Lan]

On 3/20/2020 12:07 AM, Michael Kelley wrote:
> From: Michael Kelley <mikelley@xxxxxxxxxxxxx> Sent: Thursday, March 19, 2020 8:15 AM
> > From: Tianyu Lan <Tianyu.Lan@xxxxxxxxxxxxx> Sent: Thursday, March 19, 2020 7:08 AM
> > > > > This patchset fixes some issues in the Hyper-V panic code path.
> > > > > Patch 1 resolves issue that panic system still responses network
> > > > > packets.
> > > > > Patch 2-3 resolves crash enlightenment issues.
> > > > > Patch 4 is to set crash_kexec_post_notifiers to true for Hyper-V
> > > > > VM in order to report crash data or kmsg to host before running
> > > > > kdump kernel.
> > > >
> > > > I still see an issue that isn't addressed by these patches.  The VMbus
> > > > driver registers a "die notifier" and a "panic notifier".   But die() will
> > > > eventually call panic() if panic_on_oops is set (which I think it typically
> > > > is).  If the CRASH_NOTIFY_MSG option is *not* enabled, then
> > > > hyperv_report_panic() could get called by the die notifier, and then
> > > > again by the panic notifier.
> > > >
> > > > Do we even need the "die notifier"?  If it was removed, there would
> > > > not be any notification to Hyper-V via the die() path unless panic_on_oops
> > > > is set, which I think is actually the correct behavior.  I'm not
> > > > completely clear on what is supposed to happen in general to the
> > > > Linux kernel if panic_on_oops is not set. Does it try to continue to run?
> > > > If so, then we should not be notifying Hyper-V if panic_on_oops is not
> > > > set, and removing the die notifier is the right thing to do.
> > >
> > > hyperv_report_panic() has re-enter check inside and so kernel only
> > > reports crash register data once during die().
> >
> > Ah, yes, you are right.
> >
> > >  From comment in the
> > > hyperv_report_panic(), register value reported in die chain is more
> > > exact than value in panic chain. The register value in die chain is
> > > passed by die() caller. Register value reported in panic chain
> > > is collected in the hyperv_panic_event().
> > >
> > > If panic_on_oops is not set, the task should be killed and kernel
> > > still runs. In this case, we may not trigger crash enlightenment.
> >
> > I'm not completely clear on your last statement.   It seems like there
> > is still a problem in that die() will call hyperv_report_panic() even if
> > panic_on_oops is not set.  We will have reported a panic to Hyper-V
> > even though the VM did not stop running.
Yes, the die callback is still necessary and we should skip report
if panic_on_oops isn't set.

> There's one more issue to consider.  hv_kmsg_dump() skips calling
> hyperv_report_panic_msg() if sysctl_record_panic_msg has been cleared
> by a sysctl command.  (This sysctl option gives a customer the ability to
> increase privacy by not having the VM's dmesg contents sent to Hyper-V.)
> In this case, the earlier hyperv_report_panic() call should be used.  Otherwise,
> there would not be any notification to Hyper-V about the panic.

Nice catch. I will fix this in the next version.

Thanks.