linux-kernel - Re: [PATCH RT] vfio-pci: Set MSI/MSI-X ISR to non-threaded

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Date:   Thu, 7 Dec 2017 03:52:24 -0500
From:   Steven Rostedt <rostedt@...dmis.org>
To:     "Su, David W" <david.w.su@...el.com>
Cc:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "bigeasy@...utronix.de" <bigeasy@...utronix.de>,
        "tglx@...utronix.de" <tglx@...utronix.de>
Subject: Re: [PATCH RT] vfio-pci: Set MSI/MSI-X ISR to non-threaded

On Thu, 7 Dec 2017 01:27:03 +0000
"Su, David W" <david.w.su@...el.com> wrote:

> >And spin_lock() turns into a mutex in PREEMPT_RT, which means it can
> >sleep. You can't sleep in hard interrupt context. This will eventually
> >crash the kernel.  
> 
> Steve, thanks for your review and comment.
> 
> I can think of 2 scenarios where there is contention for the eventfd
> context lock.
> 
> One scenario is an eventfd is used to notify a VFIO application of
> 2 or more IRQs.  But in this case the application wouldn't be able to
> tell which IRQ occurred and so I think it should be considered a
> programming error of the application and not a proper usage of
> VFIO.

Remember, if there is contention, the system will CRASH! Boom, panic,
end of story. Are you blowing this off to just a programming error of
the application? I'm sure lots of root kit developers are excited about
this.

> 
> The other is a device IRQ is configured to be delivered to multiple
> CPU cores at the same time.  However, I have never seen such a
> device and cannot think of any good reason for a device to be
> designed this way.
> 
> So, IMHO it is safe to set vfio-pci ISR to non-threaded.

Please tell me that you are not arguing that it's OK to have a sleeping
mutex in a hard threaded IRQ handler because "proper programming will
prevent contention". If that is truly your argument, then I'm done
here, with a perpetual NACK on your patches.

-- Steve

> 
> >
> >And no, we are not going to convert the ctx->wqh.lock into a
> >raw_spin_lock.
> >