On Thu, 1 Dec 2016, Vitaly Kuznetsov wrote:

> There is a feature in Hyper-V (Debug-VM --InjectNonMaskableInterrupt) which
> injects NMI to the guest. Prior to WS2016 the NMI is injected to all CPUs
> of the guest and WS2016 injects it to CPU0 only. When unknown_nmi_panic is
> enabled and we'd like to do kdump we need to perform some minimal cleanup
> so the kdump kernel will be able to initialize VMBus devices, this cleanup
> includes sending CHANNELMSG_UNLOAD to the host waiting for
> CHANNELMSG_UNLOAD_RESPONSE to arrive. WS2012R2 always sends the response
> to the CPU which was used to send CHANNELMSG_REQUESTOFFERS on VMBus module
> load and not to the CPU which is sending CHANNELMSG_UNLOAD. As we can't do
> any cross-CPU work reliably on crash we have vmbus_wait_for_unload()
> function which tries to read CHANNELMSG_UNLOAD_RESPONSE on all CPUs message
> pages and this sometimes works. It was discovered that in case the host
> wants to send more than one message to a secondary CPU (not the CPU running
> vmbus_wait_for_unload()) we're unable to get it as after reading the first
> message we're supposed to do EOMing by doing wrmsrl(HV_X64_MSR_EOM, 0) but
> this is per-CPU. I have a feeling that this was working some time ago when
> I implemented vmbus_wait_for_unload(), the host was re-trying to deliver a
> message even without wrmsrl() but apparently this doesn't work any more.
> Unfortunately there is not that much we can do when all CPUs get NMI as
> all but the first one are getting blocked with interrupts disabled. What we
> can do is limit processing unknown interrupts to the first CPU which gets
> it in case we're about to crash.

This is completely unreadable and I really tried hard to make sense of it.

Please structure it in a way that people who are not familiar with the
inner workings of hyperv can at least understand the problem you are trying
to solve and the concept of the solution w/o needing to figure out what all
the acronyms and constants actually mean.

Also visual structuring in paragraphs helps readability a lot.

AFAICT this tries to deal with different problems of different hypervisor
versions, but even that is unclear as you talk about version WS2016,
versions prior to WS2016 and then about WS2012R2 in particular.

Another issue I have with this is:

        ".... I have a feeling that this was working ...."

Changes like this are not about feelings. We want to have changes based on



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