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From:   "Graf (AWS), Alexander" <graf@amazon.de>
To:     Andy Lutomirski <luto@kernel.org>
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Subject: Re: [PATCH] x86/irq: Preserve vector in orig_ax for APIC code
Thread-Topic: [PATCH] x86/irq: Preserve vector in orig_ax for APIC code
Thread-Index: AQHWe9XPb47wB6d5z0uF8BBPksZN1Q==
Date:   Wed, 26 Aug 2020 18:22:09 +0000
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> Am 26.08.2020 um 20:03 schrieb Andy Lutomirski <luto@kernel.org>:
> =

>> On Wed, Aug 26, 2020 at 10:47 AM Thomas Gleixner <tglx@linutronix.de> wr=
ote:
>> =

>> 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> wr=
ote:
>>>> 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=3Dx86/entry&id=3Ddfff54208072a27909ae97ebce644c251a233ff2
>> =

>> 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=3Dx86/entry&id=3D3a5087acb8a2cc1e88b1a55fa36c2f8bef370572
>>> =

>>> 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 ho=
ld still on this patch for a bit.

Alex


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