linux-kernel - Re: [PATCH] x86/irq: Preserve vector in orig

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Date:   Wed, 26 Aug 2020 18:22:09 +0000
From:   "Graf (AWS), Alexander" <graf@...zon.de>
To:     Andy Lutomirski <luto@...nel.org>
CC:     Thomas Gleixner <tglx@...utronix.de>, X86 ML <x86@...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@...zon.de" <robketr@...zon.de>,
        Amos Kong <amos@...lladb.com>,
        "Brian Gerst" <brgerst@...il.com>, stable <stable@...r.kernel.org>
Subject: Re: [PATCH] x86/irq: Preserve vector in orig_ax for APIC code



> Am 26.08.2020 um 20:03 schrieb Andy Lutomirski <luto@...nel.org>:
> 
>> On Wed, Aug 26, 2020 at 10:47 AM Thomas Gleixner <tglx@...utronix.de> wrote:
>> 
>> Andy,
>> 
>>> On Wed, Aug 26 2020 at 09:13, Andy Lutomirski wrote:
>>> On Wed, Aug 26, 2020 at 7:27 AM Thomas Gleixner <tglx@...utronix.de> wrote:
>>>> The below nasty hack cures it, but I hate it with a passion. I'll look
>>>> deeper for a sane variant.
>>>> 
>>> Fundamentally, the way we overload orig_ax is problematic.  I have a
>>> half-written series to improve it, but my series is broken.  I think
>>> it's fixable, though.
>>> 
>>> First is this patch to use some __csh bits to indicate the entry type.
>>> As far as I know, this patch is correct:
>>> 
>>> https://git.kernel.org/pub/scm/linux/kernel/git/luto/linux.git/commit/?h=x86/entry&id=dfff54208072a27909ae97ebce644c251a233ff2
>> 
>> Yes, that looks about right.
>> 
>>> Then I wrote this incorrect patch:
>>> 
>>> https://git.kernel.org/pub/scm/linux/kernel/git/luto/linux.git/commit/?h=x86/entry&id=3a5087acb8a2cc1e88b1a55fa36c2f8bef370572
>>> 
>>> That one is wrong because the orig_ax wreckage seems to have leaked
>>> into user ABI -- user programs think that orig_ax has certain
>>> semantics on user-visible entries.
>> 
>> Yes, orig_ax is pretty much user ABI for a very long time.
>> 
>>> But I think that the problem in this thread could be fixed quite
>>> nicely by the first patch, plus a new CS_ENTRY_IRQ and allocating
>>> eight bits of __csh to store the vector.  Then we could read out the
>>> vector.
>> 
>> That works. Alternatively I can just store the vector in the irq
>> descriptor itself. That's trivial enough and can be done completely in C
>> independent of the stuff above.
> 
> The latter sounds quite sensible to me.  It does seem vaguely
> ridiculous to be trying to fish the vector out of pt_regs in the APIC
> code.

I like that option much better than the orig_ax hacks. Is this going to be something useable enough for stable?

Also, Thomas, will you have a look at moving the vector info? If so, I'd hold still on this patch for a bit.

Alex




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