Re: [PATCH] selftests/powerpc: Add script to test HMI functionality

From: Stewart Smith
Date: Wed Dec 09 2015 - 19:39:10 EST


Daniel Axtens <dja@xxxxxxxxxx> writes:
> I just realised I sent my reply to Denis not the list - apologies. This
> info goes for v2 as well.
>
> > Could you explain why it's useful, and what it's useful for. Moreover,
> > it's POWER8 feature, right?
>
> I'm not sure whether you're asking about the script or HMIs. Explaining
> HMIs helps make sense of the script, so I'll start there.
>
> HMIs are a class of interrupt or exception that, broadly speaking,
> require the hypervisor to intervene to 'do something'. They are (very
> lightly) documented in the POWER ISA, which is available on the
> OpenPOWER website. That file doesn't do a particuarly good job of
> explaining what can trigger an HMI, because that's a Book IV question.
>
> So, while I can't point you to documentation about what might cause an
> HMI, I can point you to some source code. Here goes:
>
> An HMI will (per the ISA) cause execution to jump to
> 0x0000 0000 0000 0E60. Through some asm and C you end up calling
> ppc_md.hmi_exception_early() and then possibly
> ppc_md.handle_hmi_expection(). This is only defined on PowerNV, where
> they point to opal_hmi_exception_early() and
> opal_handle_hmi_exception() respectively.
>
> The early exception calls into opal through opal_handle_hmi, which is an
> OPAL call (OPAL_HANDLE_HMI). skiboot/core/hmi.c lists the contents of
> the HMER (Hypervisor Maintenance Exception Register), which identifies
> the actual cause of the HMI. You can find the list in the skiboot repo
> on github, including the action that will be taken:
> https://github.com/open-power/skiboot/blob/master/core/hmi.c
> The rest of the file fleshes out the mechanics of HMIs: for example,
> where they are caused by the failure of a POWER8 co-processor such as
> CAPI or NX.
>
> Some HMIs are relayed by Skiboot to Linux by sending an OPAL_MSG_HMI_EVT
> to Linux. This triggers off some further processing which causes a
> message to be printed in dmesg. The relevant file here is
> platforms/powernv/opal-hmi.c
>
> The script, therefore, is useful because:
> - HMIs are an exceptional/error condition that is not hit in normal
> operation. Indeed, without the xscom commands in this script
> (or a CAPI card), it's almost impossible to hit them.
> - HMIs involve communications between Skiboot and Linux, involve
> touching the PACA, and generally work in an area that is prone to
> bugs, so testing them is especially valuable.
> - The script is carefully calibrated to send HMIs that trigger a
> message in dmesg but which don't checkstop the machine.
>
> To answer your final question, I'm not entirely sure if HMIs are POWER8
> specific. I suspect they've been around for a lot longer, but maybe
> someone who's been around IBM chips for longer than me could clarify this.

Adding this to doc/ somewhere in kernel and/or skiboot would be
great. There's a skiboot doc/hmi.txt that's begging for a patch, you
know, creating it :)

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