linux-kernel - RE: x86/irq: Unbreak interrupt affinity setting

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Message-ID: <87ft89kqmd.fsf@nanos.tec.linutronix.de>
Date:   Thu, 27 Aug 2020 00:07:54 +0200
From:   Thomas Gleixner <tglx@...utronix.de>
To:     David Laight <David.Laight@...LAB.COM>,
        Alexander Graf <graf@...zon.com>, X86 ML <x86@...nel.org>
Cc:     Andy Lutomirski <luto@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Andrew Cooper <andrew.cooper3@...rix.com>,
        "Paul E. McKenney" <paulmck@...nel.org>,
        Alexandre Chartre <alexandre.chartre@...cle.com>,
        Frederic Weisbecker <frederic@...nel.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Sean Christopherson <sean.j.christopherson@...el.com>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Petr Mladek <pmladek@...e.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Joel Fernandes <joel@...lfernandes.org>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Juergen Gross <jgross@...e.com>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Will Deacon <will@...nel.org>,
        Tom Lendacky <thomas.lendacky@....com>,
        Wei Liu <wei.liu@...nel.org>,
        Michael Kelley <mikelley@...rosoft.com>,
        Jason Chen CJ <jason.cj.chen@...el.com>,
        Zhao Yakui <yakui.zhao@...el.com>,
        "Peter Zijlstra \(Intel\)" <peterz@...radead.org>,
        Avi Kivity <avi@...lladb.com>,
        "Herrenschmidt\, Benjamin" <benh@...zon.com>,
        "robketr\@amazon.de" <robketr@...zon.de>,
        "amos\@scylladb.com" <amos@...lladb.com>,
        Brian Gerst <brgerst@...il.com>,
        "stable\@vger.kernel.org" <stable@...r.kernel.org>,
        Alex bykov <alex.bykov@...lladb.com>
Subject: RE: x86/irq: Unbreak interrupt affinity setting

On Wed, Aug 26 2020 at 21:37, David Laight wrote:
> From: Thomas Gleixner
>> Sent: 26 August 2020 21:22
> ...
>> Moving interrupts on x86 happens in several steps. A new vector on a
>> different CPU is allocated and the relevant interrupt source is
>> reprogrammed to that. But that's racy and there might be an interrupt
>> already in flight to the old vector. So the old vector is preserved until
>> the first interrupt arrives on the new vector and the new target CPU. Once
>> that happens the old vector is cleaned up, but this cleanup still depends
>> on the vector number being stored in pt_regs::orig_ax, which is now -1.
>
> I suspect that it is much more 'racy' than that for PCI-X interrupts.
> On the hardware side there is an interrupt disable bit, and address
> and a value.
> To raise an interrupt the hardware must write the value to the
> address.

Really?

> If the cpu needs to move an interrupt both the address and value
> need changing, but the cpu wont write the address and value using
> the same TLP, so the hardware could potentially write a value to
> the wrong address.

Now I understand finally why msi_set_affinity() in x86 has to be so
convoluted.

Thanks a lot for the enlightment!

       tglx