linux-kernel - [RFC PATCH 5/6] x86/fpu: Fix CPUID-less FPU detection

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Date:   Mon,  5 Dec 2016 17:01:14 -0800
From:   Andy Lutomirski <luto@...nel.org>
To:     x86@...nel.org
Cc:     One Thousand Gnomes <gnomes@...rguk.ukuu.org.uk>,
        Borislav Petkov <bp@...en8.de>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Brian Gerst <brgerst@...il.com>,
        Matthew Whitehead <tedheadster@...il.com>,
        Andy Lutomirski <luto@...nel.org>
Subject: [RFC PATCH 5/6] x86/fpu: Fix CPUID-less FPU detection

The old code didn't work at all because it adjusted the current caps
instead of the forced caps.  Anything it did would be undone later
during cpu identification.  Fix that and, while we're at it, improve
the logging and don't bother running it if CPUID is available.

Signed-off-by: Andy Lutomirski <luto@...nel.org>
---
 arch/x86/kernel/fpu/init.c | 28 ++++++++++++++++------------
 1 file changed, 16 insertions(+), 12 deletions(-)

diff --git a/arch/x86/kernel/fpu/init.c b/arch/x86/kernel/fpu/init.c
index 60dece392b3a..75e1bf3b0319 100644
--- a/arch/x86/kernel/fpu/init.c
+++ b/arch/x86/kernel/fpu/init.c
@@ -50,29 +50,33 @@ void fpu__init_cpu(void)
 
 /*
  * The earliest FPU detection code.
- *
- * Set the X86_FEATURE_FPU CPU-capability bit based on
- * trying to execute an actual sequence of FPU instructions:
  */
 static void fpu__init_system_early_generic(struct cpuinfo_x86 *c)
 {
-	unsigned long cr0;
-	u16 fsw, fcw;
+	if (!boot_cpu_has(X86_FEATURE_CPUID) &&
+	    !test_bit(X86_FEATURE_FPU, (unsigned long *)cpu_caps_cleared)) {
+		/*
+		 * Set the X86_FEATURE_FPU CPU-capability bit based on
+		 * trying to execute an actual sequence of FPU instructions:
+		 */
 
-	fsw = fcw = 0xffff;
+		unsigned long cr0;
+		u16 fsw, fcw;
 
-	cr0 = read_cr0();
-	cr0 &= ~(X86_CR0_TS | X86_CR0_EM);
-	write_cr0(cr0);
+		fsw = fcw = 0xffff;
+
+		cr0 = read_cr0();
+		cr0 &= ~(X86_CR0_TS | X86_CR0_EM);
+		write_cr0(cr0);
 
-	if (!test_bit(X86_FEATURE_FPU, (unsigned long *)cpu_caps_cleared)) {
 		asm volatile("fninit ; fnstsw %0 ; fnstcw %1"
 			     : "+m" (fsw), "+m" (fcw));
+		pr_info("x86/fpu: FSW=0x%04hx FCW=0x%04hx\n", fsw, fcw);
 
 		if (fsw == 0 && (fcw & 0x103f) == 0x003f)
-			set_cpu_cap(c, X86_FEATURE_FPU);
+			setup_force_cpu_cap(X86_FEATURE_FPU);
 		else
-			clear_cpu_cap(c, X86_FEATURE_FPU);
+			setup_clear_cpu_cap(X86_FEATURE_FPU);
 	}
 
 #ifndef CONFIG_MATH_EMULATION
-- 
2.9.3