linux-kernel - RE: [PATCH] x86/entry/64: Fix native_load_gs_index() SWAPGS handling with IRQ state tracing enabled

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Date:   Wed, 29 Nov 2017 16:51:20 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Andy Lutomirski' <luto@...nel.org>
CC:     Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...nel.org>,
        Jarkko Nikula <jarkko.nikula@...ux.intel.com>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        "Linus Torvalds" <torvalds@...ux-foundation.org>,
        Borislav Petkov <bp@...en8.de>
Subject: RE: [PATCH] x86/entry/64: Fix native_load_gs_index() SWAPGS handling
 with IRQ state tracing enabled

From: Andy Lutomirski
> Sent: 29 November 2017 16:23
> > I can't remember what happens when swapgs itself faults.
...
> SWAPGS never faults.

Ah yes, I remember, it only restores the offset.
For 32bit processes you need to do a 'pop %gs' to recover
the segment register itself - and that can fault.

SWAPGS is a PITA. The hardware designers should have tried to write
all the interrupt handling code.

	David