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From:   Thomas Gleixner <tglx@linutronix.de>
To:     Andy Lutomirski <luto@kernel.org>
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        stable <stable@vger.kernel.org>
Subject: Re: [PATCH] x86/irq: Preserve vector in orig_ax for APIC code
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References: <20200826115357.3049-1-graf@amazon.com> <87k0xlv5w5.fsf@nanos.tec.linutronix.de> <CALCETrX-8a61k03+XJop=k11-TkE+7JOiGTH=81sHXPmXsA+Tw@mail.gmail.com>
Date:   Wed, 26 Aug 2020 19:47:39 +0200
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Andy,

On Wed, Aug 26 2020 at 09:13, Andy Lutomirski wrote:
> On Wed, Aug 26, 2020 at 7:27 AM Thomas Gleixner <tglx@linutronix.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.

Thanks,

        tglx