[PATCH][xenial linux] x86/fpu: Set X86_FEATURE_OSXSAVE feature after enabling OSXSAVE in CR4

[Tim Gardner]

From: Feng Tang <feng.tang at intel.com>

0-Day found a 34.6% regression in stress-ng's 'af-alg' test case, and
bisected it to commit b81fac906a8f ("x86/fpu: Move FPU initialization into
arch_cpu_finalize_init()"), which optimizes the FPU init order, and moves
the CR4_OSXSAVE enabling into a later place:

   arch_cpu_finalize_init
       identify_boot_cpu
	   identify_cpu
	       generic_identify
                   get_cpu_cap --> setup cpu capability
       ...
       fpu__init_cpu
           fpu__init_cpu_xstate
               cr4_set_bits(X86_CR4_OSXSAVE);

As the FPU is not yet initialized the CPU capability setup fails to set
X86_FEATURE_OSXSAVE. Many security module like 'camellia_aesni_avx_x86_64'
depend on this feature and therefore fail to load, causing the regression.

Cure this by setting X86_FEATURE_OSXSAVE feature right after OSXSAVE
enabling.

[ tglx: Moved it into the actual BSP FPU initialization code and added a comment ]

Fixes: b81fac906a8f ("x86/fpu: Move FPU initialization into arch_cpu_finalize_init()")
Reported-by: kernel test robot <oliver.sang at intel.com>
Signed-off-by: Feng Tang <feng.tang at intel.com>
Signed-off-by: Thomas Gleixner <tglx at linutronix.de>
Cc: stable at vger.kernel.org
Link: https://lore.kernel.org/lkml/202307192135.203ac24e-oliver.sang@intel.com
Link: https://lore.kernel.org/lkml/20230823065747.92257-1-feng.tang@intel.com
(backported from commit 2c66ca3949dc701da7f4c9407f2140ae425683a5)
[rtg - context adjustment]
Signed-off-by: Tim Gardner <tim.gardner at canonical.com>
---
 arch/x86/kernel/fpu/xstate.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/arch/x86/kernel/fpu/xstate.c b/arch/x86/kernel/fpu/xstate.c
index 3fa200ecca62..b30b53c7a648 100644
--- a/arch/x86/kernel/fpu/xstate.c
+++ b/arch/x86/kernel/fpu/xstate.c
@@ -648,6 +648,13 @@ void __init fpu__init_system_xstate(void)
 	setup_init_fpu_buf();
 	setup_xstate_comp();
 
+	/*
+	 * CPU capabilities initialization runs before FPU init. So
+	 * X86_FEATURE_OSXSAVE is not set. Now that XSAVE is completely
+	 * functional, set the feature bit so depending code works.
+	 */
+	setup_force_cpu_cap(X86_FEATURE_OSXSAVE);
+
 	pr_info("x86/fpu: Enabled xstate features 0x%llx, context size is %d bytes, using '%s' format.\n",
 		xfeatures_mask,
 		xstate_size,
-- 
2.34.1