linux-kernel - [PATCH] x86, fpu: do not BUG

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Date:	Wed, 20 Jul 2016 09:23:28 -0700
From:	Dave Hansen <dave@...1.net>
To:	linux-kernel@...r.kernel.org
Cc:	x86@...nel.org, Dave Hansen <dave@...1.net>,
	dave.hansen@...ux.intel.com
Subject: [PATCH] x86, fpu: do not BUG_ON() in early FPU code

From: Dave Hansen <dave.hansen@...ux.intel.com>

I don't think it is really possible to have a system where CPUID
enumerates support for XSAVE but that it does not have FP/SSE
(they are "legacy" features and always present).

But, I did manage to hit this case in qemu when I enabled its
somewhat shaky XSAVE support.  The bummer is that the FPU is set
up before we parse the command-line or have *any* console support
including earlyprintk.

So a BUG() here is worthless.  All it does it guarantee that
if/when we hit this case we have an empty console.  So, remove
the BUG() and try to limp along by disabling XSAVE and trying to
continue.

Signed-off-by: Dave Hansen <dave.hansen@...ux.intel.com>
---

 b/arch/x86/kernel/fpu/xstate.c |    3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff -puN arch/x86/kernel/fpu/xstate.c~x86-fpu-do-not-BUG_ON-early arch/x86/kernel/fpu/xstate.c
--- a/arch/x86/kernel/fpu/xstate.c~x86-fpu-do-not-BUG_ON-early	2016-07-20 09:03:07.347941392 -0700
+++ b/arch/x86/kernel/fpu/xstate.c	2016-07-20 09:17:15.001309137 -0700
@@ -715,7 +715,8 @@ void __init fpu__init_system_xstate(void

 	if ((xfeatures_mask & XFEATURE_MASK_FPSSE) != XFEATURE_MASK_FPSSE) {
 		pr_err("x86/fpu: FP/SSE not present amongst the CPU's xstate features: 0x%llx.\n", xfeatures_mask);
-		BUG();
+		fpu__init_disable_system_xstate();
+		return;
 	}

 	xfeatures_mask &= fpu__get_supported_xfeatures_mask();
_