linux-kernel - Re: [PATCH 2/4] x86/asm/entry: Untangle 'ia32_sysenter_target' into two entry points: entry_SYSENTER_32 and entry_SYSENTER

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Date:	Mon, 8 Jun 2015 17:13:21 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	Ingo Molnar <mingo@...nel.org>
Cc:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Denys Vlasenko <dvlasenk@...hat.com>,
	Brian Gerst <brgerst@...il.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Borislav Petkov <bp@...en8.de>,
	"H. Peter Anvin" <hpa@...or.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Oleg Nesterov <oleg@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH 2/4] x86/asm/entry: Untangle 'ia32_sysenter_target' into
 two entry points: entry_SYSENTER_32 and entry_SYSENTER_compat

On Mon, Jun 8, 2015 at 1:34 AM, Ingo Molnar <mingo@...nel.org> wrote:
> So the SYSENTER instruction is pretty quirky and it has different behavior
> depending on bitness and CPU maker.
>
> Yet we create a false sense of coherency by naming it 'ia32_sysenter_target'
> in both of the cases.
>
> Split the name into its two uses:
>
>         ia32_sysenter_target (32)    -> entry_SYSENTER_32
>         ia32_sysenter_target (64)    -> entry_SYSENTER_compat
>

Now that I'm rebasing my pile on top of this, I have a minor gripe
about this one.  There are (in my mind, anyway), two SYSENTER
instructions: the 32-bit one and the 64-bit one.  (That is, there's
SYSENTER32, which happens when you do SYSENTER in 32-bit or compat
mode, and SYSENTER64, which happens when you do SYSENTER in long
mode.)  SYSENTER32, from user code's perspective, does the same thing
in either case [1].  That means that it really does make sense that
we'd have two implementations of the same entry point, one written in
32-bit asm and one written in 64-bit asm.

The patch I'm rebasing merges the two wrmsrs to MSR_IA32_SYSENTER, and
this change makes it uglier.

[1] Sort of.  We probably have differently nonsensical calling
conventions, but that's our fault and has nothing to do with the
hardware.

--Andy
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