Re: Should arm64 have a custom crash shutdown handler?

[Vitaly Kuznetsov]

Mark Rutland <mark.rutland@xxxxxxx> writes:

> On Thu, May 05, 2022 at 03:52:24PM +0200, Vitaly Kuznetsov wrote:
>> "Guilherme G. Piccoli" <gpiccoli@xxxxxxxxxx> writes:
>> 
>> > On 05/05/2022 09:53, Mark Rutland wrote:
>> >> [...]
>> >> Looking at those, the cleanup work is all arch-specific. What exactly would we
>> >> need to do on arm64, and why does it need to happen at that point specifically?
>> >> On arm64 we don't expect as much paravirtualization as on x86, so it's not
>> >> clear to me whether we need anything at all.
>> >> 
>> >>> Anyway, the idea here was to gather a feedback on how "receptive" arm64
>> >>> community would be to allow such customization, appreciated your feedback =)
>> >> 
>> >> ... and are you trying to do this for Hyper-V or just using that as an example?
>> >> 
>> >> I think we're not going to be very receptive without a more concrete example of
>> >> what you want.
>> >> 
>> >> What exactly do *you* need, and *why*? Is that for Hyper-V or another hypervisor?
>> >> 
>> >> Thanks
>> >> Mark.
>> >
>> > Hi Mark, my plan would be doing that for Hyper-V - kind of the same
>> > code, almost. For example, in hv_crash_handler() there is a stimer
>> > clean-up and the vmbus unload - my understanding is that this same code
>> > would need to run in arm64. Michael Kelley is CCed, he was discussing
>> > with me in the panic notifiers thread and may elaborate more on the needs.
>> >
>> > But also (not related with my specific plan), I've seen KVM quiesce code
>> > on x86 as well [see kvm_crash_shutdown() on arch/x86] , I'm not sure if
>> > this is necessary for arm64 or if this already executing in some
>> > abstracted form, I didn't dig deep - probably Vitaly is aware of that,
>> > hence I've CCed him here.
>> 
>> Speaking about the difference between reboot notifiers call chain and
>> machine_ops.crash_shutdown for KVM/x86, the main difference is that
>> reboot notifier is called on some CPU while the VM is fully functional,
>> this way we may e.g. still use IPIs (see kvm_pv_reboot_notify() doing
>> on_each_cpu()). When we're in a crash situation,
>> machine_ops.crash_shutdown is called on the CPU which crashed. We can't
>> count on IPIs still being functional so we do the very basic minimum so
>> *this* CPU can boot kdump kernel. There's no guarantee other CPUs can
>> still boot but normally we do kdump with 'nprocs=1'.
>
> Sure; IIUC the IPI problem doesn't apply to arm64, though, since that doesn't
> use a PV mechanism (and practically speaking will either be GICv2 or GICv3).
>

This isn't really about PV: when the kernel is crashing, you have no
idea what's going on on other CPUs, they may be crashing too, locked in
a tight loop, ... so sending an IPI there to do some work and expecting
it to report back is dangerous.

>> For Hyper-V, the situation is similar: hv_crash_handler() intitiates
>> VMbus unload on the crashing CPU only, there's no mechanism to do
>> 'global' unload so other CPUs will likely not be able to connect Vmbus
>> devices in kdump kernel but this should not be necessary.
>
> Given kdump is best-effort (and we can't rely on secondary CPUs even making it
> into the kdump kernel), I also don't think that should be necessary.

Yes, exactly.

>
>> There's a crash_kexec_post_notifiers mechanism which can be used instead
>> but it's disabled by default so using machine_ops.crash_shutdown is
>> better.
>
> Another option is to defer this to the kdump kernel. On arm64 at least, we know
> if we're in a kdump kernel early on, and can reset some state based upon that.
>
> Looking at x86's hyperv_cleanup(), everything relevant to arm64 can be deferred
> to just before the kdump kernel detects and initializes anything relating to
> hyperv. So AFAICT we could have hyperv_init() check is_kdump_kernel() prior to
> the first hypercall, and do the cleanup/reset there.

In theory yes, it is possible to try sending CHANNELMSG_UNLOAD on kdump
kernel boot and not upon crash, I don't remember if this approach was
tried in the past. 

>
> Maybe we need more data for the vmbus bits? ... if so it seems that could blow
> up anyway when the first kernel was tearing down.

Not sure I understood what you mean... From what I remember, there were
issues with CHANNELMSG_UNLOAD handling on the Hyper-V host side in the
past (it was taking *minutes* for the host to reply) but this is
orthogonal to the fact that we need to do this cleanup so kdump kernel
is able to connect to Vmbus devices again.

-- 
Vitaly