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From: David Laight <David.Laight@ACULAB.COM>
To: "'Andy Lutomirski'" <luto@kernel.org>
CC: Peter Zijlstra <peterz@infradead.org>, Ingo Molnar <mingo@kernel.org>,
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Subject: RE: [PATCH] x86/entry/64: Fix native_load_gs_index() SWAPGS handling
 with IRQ state tracing enabled
Thread-Topic: [PATCH] x86/entry/64: Fix native_load_gs_index() SWAPGS handling
 with IRQ state tracing enabled
Thread-Index: AQHTaR8fKskq+GLGsUC5ju1OFMHHkKMrb/rwgAAaX4CAAAE0cA==
Date: Wed, 29 Nov 2017 16:51:20 +0000
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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