linux-kernel - Re: [RFC/HACK] x86: Fast return to kernel

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Date:	Fri, 2 May 2014 14:42:05 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	"H. Peter Anvin" <h.peter.anvin@...el.com>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	"the arch/x86 maintainers" <x86@...nel.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Gleb Natapov <gleb@...nel.org>,
	Paolo Bonzini <pbonzini@...hat.com>
Subject: Re: [RFC/HACK] x86: Fast return to kernel

On Fri, May 2, 2014 at 2:37 PM, H. Peter Anvin <h.peter.anvin@...el.com> wrote:
> On 05/02/2014 02:07 PM, Linus Torvalds wrote:
>> On Fri, May 2, 2014 at 2:04 PM, Andy Lutomirski <luto@...capital.net> wrote:
>>>
>>> Because otherwise I'd have to keep track of whether it's a zeroentry
>>> or an errorentry.  I can't stuff the offset in a register without even
>>> more stack hackery, since there are no available registers there.  I
>>> could split the whole thing into two code paths, I guess.
>>
>> Ahh. Never mind. I didn't think about the fact that the error entry
>> case had one more field on the stack. Your approach is all fine, it
>> was me not seeing the problem.
>>
>
> I have to admit to being rather partial to the idea of simply doing
> "push $0" on entry for the vectors that don't push an error code, like
> the early exception handling code does.
>

Hah -- I think I just faked both of you out :)

I don't think this has anything to do with the error code, and I think
that the errorentry code already does more or less that: it pushes -1.

The real issue here is probably the magic 16-byte stack alignment when
a non-stack-switching interrupt happens.

--Andy
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