x86/fpu: Revert ("x86/fpu: Disable AVX when eagerfpu is off")

yyu-intel-com · Ingo Molnar · commit a65050c6f17e · 2016-03-10T10:15:58.000+01:00
Leonid Shatz noticed that the SDM interpretation of the following recent commit: 394db20 ("x86/fpu: Disable AVX when eagerfpu is off") ... is incorrect and that the original behavior of the FPU code was correct. Because AVX is not stated in CR0 TS bit description, it was mistakenly believed to be not supported for lazy context switch. This turns out to be false: Intel Software Developer's Manual Vol. 3A, Sec. 2.5 Control Registers: 'TS Task Switched bit (bit 3 of CR0) -- Allows the saving of the x87 FPU/ MMX/SSE/SSE2/SSE3/SSSE3/SSE4 context on a task switch to be delayed until an x87 FPU/MMX/SSE/SSE2/SSE3/SSSE3/SSE4 instruction is actually executed by the new task.' Intel Software Developer's Manual Vol. 2A, Sec. 2.4 Instruction Exception Specification: 'AVX instructions refer to exceptions by classes that include #NM "Device Not Available" exception for lazy context switch.' So revert the commit. Reported-by: Leonid Shatz <leonid.shatz@ravellosystems.com> Signed-off-by: Yu-cheng Yu <yu-cheng.yu@intel.com> Cc: Andy Lutomirski <luto@kernel.org> Cc: Borislav Petkov <bp@suse.de> Cc: Dave Hansen <dave.hansen@linux.intel.com> Cc: Fenghua Yu <fenghua.yu@intel.com> Cc: Linus Torvalds <torvalds@linux-foundation.org> Cc: Peter Zijlstra <peterz@infradead.org> Cc: Ravi V. Shankar <ravi.v.shankar@intel.com> Cc: Sai Praneeth Prakhya <sai.praneeth.prakhya@intel.com> Cc: Thomas Gleixner <tglx@linutronix.de> Link: http://lkml.kernel.org/r/1457569734-3785-1-git-send-email-yu-cheng.yu@intel.com Signed-off-by: Ingo Molnar <mingo@kernel.org>
diff --git a/arch/x86/include/asm/fpu/xstate.h b/arch/x86/include/asm/fpu/xstate.h
@@ -20,16 +20,15 @@
 
 /* Supported features which support lazy state saving */
 #define XFEATURE_MASK_LAZY	(XFEATURE_MASK_FP | \
-				 XFEATURE_MASK_SSE)
-
-/* Supported features which require eager state saving */
-#define XFEATURE_MASK_EAGER	(XFEATURE_MASK_BNDREGS | \
-				 XFEATURE_MASK_BNDCSR | \
+				 XFEATURE_MASK_SSE | \
 				 XFEATURE_MASK_YMM | \
 				 XFEATURE_MASK_OPMASK | \
 				 XFEATURE_MASK_ZMM_Hi256 | \
 				 XFEATURE_MASK_Hi16_ZMM)
 
+/* Supported features which require eager state saving */
+#define XFEATURE_MASK_EAGER	(XFEATURE_MASK_BNDREGS | XFEATURE_MASK_BNDCSR)
+
 /* All currently supported features */
 #define XCNTXT_MASK	(XFEATURE_MASK_LAZY | XFEATURE_MASK_EAGER)
 
diff --git a/arch/x86/kernel/fpu/init.c b/arch/x86/kernel/fpu/init.c
@@ -302,12 +302,6 @@ u64 __init fpu__get_supported_xfeatures_mask(void)
 static void __init fpu__clear_eager_fpu_features(void)
 {
 	setup_clear_cpu_cap(X86_FEATURE_MPX);
-	setup_clear_cpu_cap(X86_FEATURE_AVX);
-	setup_clear_cpu_cap(X86_FEATURE_AVX2);
-	setup_clear_cpu_cap(X86_FEATURE_AVX512F);
-	setup_clear_cpu_cap(X86_FEATURE_AVX512PF);
-	setup_clear_cpu_cap(X86_FEATURE_AVX512ER);
-	setup_clear_cpu_cap(X86_FEATURE_AVX512CD);
 }
 
 /*

Original file line number	Diff line number	Diff line change
`@@ -302,12 +302,6 @@ u64 __init fpu__get_supported_xfeatures_mask(void)`
`302`	`302`	`static void __init fpu__clear_eager_fpu_features(void)`
`303`	`303`	`{`
`304`	`304`	`setup_clear_cpu_cap(X86_FEATURE_MPX);`
`305`		`- setup_clear_cpu_cap(X86_FEATURE_AVX);`
`306`		`- setup_clear_cpu_cap(X86_FEATURE_AVX2);`
`307`		`- setup_clear_cpu_cap(X86_FEATURE_AVX512F);`
`308`		`- setup_clear_cpu_cap(X86_FEATURE_AVX512PF);`
`309`		`- setup_clear_cpu_cap(X86_FEATURE_AVX512ER);`
`310`		`- setup_clear_cpu_cap(X86_FEATURE_AVX512CD);`
`311`	`305`	`}`
`312`	`306`
`313`	`307`	`/*`