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From:   Andy Lutomirski <luto@kernel.org>
Date:   Wed, 26 Aug 2020 11:00:23 -0700
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Subject: Re: [PATCH] x86/irq: Preserve vector in orig_ax for APIC code
To:     Thomas Gleixner <tglx@linutronix.de>
Cc:     Andy Lutomirski <luto@kernel.org>,
        Alexander Graf <graf@amazon.com>, X86 ML <x86@kernel.org>,
        LKML <linux-kernel@vger.kernel.org>,
        Andrew Cooper <andrew.cooper3@citrix.com>,
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On Wed, Aug 26, 2020 at 10:47 AM Thomas Gleixner <tglx@linutronix.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@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.

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.

--Andy