linux-kernel - Re: [PATCH v3] s390/pci: fix CPU address in MSI for directed IRQ

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Message-ID: <20201130095549.27da927f.pasic@linux.ibm.com>
Date:   Mon, 30 Nov 2020 09:55:49 +0100
From:   Halil Pasic <pasic@...ux.ibm.com>
To:     Niklas Schnelle <schnelle@...ux.ibm.com>
Cc:     Alexander Gordeev <agordeev@...ux.ibm.com>,
        linux-s390@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3] s390/pci: fix CPU address in MSI for directed IRQ

On Mon, 30 Nov 2020 09:30:33 +0100
Niklas Schnelle <schnelle@...ux.ibm.com> wrote:

> I'm not really familiar, with it but I think this is closely related
> to what I asked Bernd Nerz. I fear that if CPUs go away we might already
> be in trouble at the firmware/hardware/platform level because the CPU Address is
> "programmed into the device" so to speak. Thus a directed interrupt from
> a device may race with anything reordering/removing CPUs even if
> CPU addresses of dead CPUs are not reused and the mapping is stable.

From your answer, I read that CPU hot-unplug is supported for LPAR. 
> 
> Furthermore our floating fallback path will try to send a SIGP
> to the target CPU which clearly doesn't work when that is permanently
> gone. Either way I think these issues are out of scope for this fix
> so I will go ahead and merge this.

I agree, it makes on sense to delay this fix.

But if CPU hot-unplug is supported, I believe we should react when
a CPU is unplugged, that is a target of directed interrupts. My guess
is, that in this scenario transient hiccups are unavoidable, and thus
should be accepted, but we should make sure that we recover.

Regards,
Halil