linux-kernel - [RFC PATCH] x86/boot: Initialize FPU and X86_FEATURE

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Date:   Wed, 28 Sep 2016 16:06:33 -0700
From:   Andy Lutomirski <luto@...nel.org>
To:     x86@...nel.org
Cc:     david@...giorato.net, Borislav Petkov <bp@...en8.de>,
        Brian Gerst <brgerst@...il.com>, linux-kernel@...r.kernel.org,
        Andy Lutomirski <luto@...nel.org>, stable@...r.kernel.org,
        Dave Hansen <dave@...1.net>, Ingo Molnar <mingo@...nel.org>
Subject: [RFC PATCH] x86/boot: Initialize FPU and X86_FEATURE_ALWAYS even if we don't have CPUID

Otherwise arch_task_struct_size == 0 and we die.  While we're at it,
set X86_FEATURE_ALWAYS, too.

Cc: stable@...r.kernel.org
Cc: Dave Hansen <dave@...1.net>
Cc: Ingo Molnar <mingo@...nel.org>
Fixes: aaeb5c01c5b ("x86/fpu, sched: Introduce CONFIG_ARCH_WANTS_DYNAMIC_TASK_STRUCT and use it on x86")
Reported-by: david@...giorato.net
Signed-off-by: Andy Lutomirski <luto@...nel.org>
---

Totally untested except by the 0-day bot, since I don't have an
affected machine and QEMU can't emulate anything without CPUID.

 arch/x86/kernel/cpu/common.c | 23 +++++++++++------------
 1 file changed, 11 insertions(+), 12 deletions(-)

diff --git a/arch/x86/kernel/cpu/common.c b/arch/x86/kernel/cpu/common.c
index 809eda03c527..bcc9ccc220c9 100644
--- a/arch/x86/kernel/cpu/common.c
+++ b/arch/x86/kernel/cpu/common.c
@@ -804,21 +804,20 @@ static void __init early_identify_cpu(struct cpuinfo_x86 *c)
 		identify_cpu_without_cpuid(c);
 
 	/* cyrix could have cpuid enabled via c_identify()*/
-	if (!have_cpuid_p())
-		return;
+	if (have_cpuid_p()) {
+		cpu_detect(c);
+		get_cpu_vendor(c);
+		get_cpu_cap(c);
 
-	cpu_detect(c);
-	get_cpu_vendor(c);
-	get_cpu_cap(c);
-
-	if (this_cpu->c_early_init)
-		this_cpu->c_early_init(c);
+		if (this_cpu->c_early_init)
+			this_cpu->c_early_init(c);
 
-	c->cpu_index = 0;
-	filter_cpuid_features(c, false);
+		c->cpu_index = 0;
+		filter_cpuid_features(c, false);
 
-	if (this_cpu->c_bsp_init)
-		this_cpu->c_bsp_init(c);
+		if (this_cpu->c_bsp_init)
+			this_cpu->c_bsp_init(c);
+	}
 
 	setup_force_cpu_cap(X86_FEATURE_ALWAYS);
 	fpu__init_system(c);
-- 
2.7.4