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

[Ingo Molnar <mingo@kernel.org>]

* Jarkko Nikula <jarkko.nikula@linux.intel.com> wrote:

> Hi
> 
> Suspend-to-ram and resume stopped working on v4.15-rc1 and I bisected it to
> commit ca37e57bbe0c ("x86/entry/64: Add missing irqflags tracing to
> native_load_gs_index()").
> 
> I noticed it on Intel Kabylake (core) and Apollolake (atom) based prototype
> machines. Symptoms are that machine appears to enter into suspend but
> resumes instantly and hangs. Unfortunately no logs.
> 
> If I revert ca37e57bbe0c on v4.15-rc1 it works as expected.

Hm, that commit looks broken with irq-tracing enabled.
Does the patch below fix it?

In fact the exception handler itself appears to have broken GS handling as well - 
I suspect it never triggers in practice, because it was broken forever.

Andy, do you concur?

On a related note, we should definitely extend the 'intended GS state' annotation 
comments I did in this patch to all SWAPGS instances - this way code review has a 
much higher chance of finding discrepancies between intent and actual code.

Thanks,

	Ingo

=================>
>From 769dbd33a272214c48c0fc5a17bed9c1597e222f Mon Sep 17 00:00:00 2001
From: Ingo Molnar <mingo@kernel.org>
Date: Wed, 29 Nov 2017 07:43:27 +0100
Subject: [PATCH] x86/entry/64: Fix native_load_gs_index() SWAPGS handling with IRQ state tracing enabled

Jarkko Nikula reported a S2R resume hang regression and bisected it back to:

  ca37e57bbe0c ("x86/entry/64: Add missing irqflags tracing to native_load_gs_index()")

Turns out the GS handling of that patch is wrong: when IRQ state tracing is
enabled it calls a kernel function (as part of the TRACE_IRQS_*() functionality),
but we have not switched to the kernel GS yet ...

Fix the SWAPGS handling and also annotate every affected SWAPGS
instance to document the intended state of GS.

Reported-by: Jarkko Nikula <jarkko.nikula@linux.intel.com>
Bisected-by: Jarkko Nikula <jarkko.nikula@linux.intel.com>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/0fede9f9-88b0-a6e7-1027-dfb2019b8ef2@linux.intel.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>
---
 arch/x86/entry/entry_64.S | 7 ++++---
 1 file changed, 4 insertions(+), 3 deletions(-)

diff --git a/arch/x86/entry/entry_64.S b/arch/x86/entry/entry_64.S
index f81d50d7ceac..c0b52df8ee4f 100644
--- a/arch/x86/entry/entry_64.S
+++ b/arch/x86/entry/entry_64.S
@@ -945,16 +945,16 @@ idtentry simd_coprocessor_error		do_simd_coprocessor_error	has_error_code=0
 	 */
 ENTRY(native_load_gs_index)
 	FRAME_BEGIN
+	SWAPGS					/* switch from user GS to kernel GS */
 	pushfq
 	DISABLE_INTERRUPTS(CLBR_ANY & ~CLBR_RDI)
 	TRACE_IRQS_OFF
-	SWAPGS
 .Lgs_change:
 	movl	%edi, %gs
 2:	ALTERNATIVE "", "mfence", X86_BUG_SWAPGS_FENCE
-	SWAPGS
 	TRACE_IRQS_FLAGS (%rsp)
 	popfq
+	SWAPGS					/* switch from kernel GS to user GS */
 	FRAME_END
 	ret
 ENDPROC(native_load_gs_index)
@@ -964,7 +964,7 @@ EXPORT_SYMBOL(native_load_gs_index)
 	.section .fixup, "ax"
 	/* running with kernelgs */
 bad_gs:
-	SWAPGS					/* switch back to user gs */
+	SWAPGS					/* switch back to user GS, to modify GS */
 .macro ZAP_GS
 	/* This can't be a string because the preprocessor needs to see it. */
 	movl $__USER_DS, %eax
@@ -973,6 +973,7 @@ EXPORT_SYMBOL(native_load_gs_index)
 	ALTERNATIVE "", "ZAP_GS", X86_BUG_NULL_SEG
 	xorl	%eax, %eax
 	movl	%eax, %gs
+	SWAPGS					/* switch to kernel GS again before continuing */
 	jmp	2b
 	.previous