linux_5_0: restore __kernel_fpu_{begin,restore}

In 5.0er these function were removed from the public interface also zfs needs
them for AVX/AES-NI support. Without this patch for example throughput on a
encrypted zfs dataset drops to 200 MB/s from 1.2 GB/s. These functions were
removed as their was no user within the linux kernel tree itself.

pkgs/os-specific/linux/kernel/export_kernel_fpu_functions.patch

@@ -0,0 +1,60 @@
+From 245e0f743d814c9ff2d1c748175e321301eb16cf Mon Sep 17 00:00:00 2001
+From: =?UTF-8?q?J=C3=B6rg=20Thalheim?= <joerg@thalheim.io>
+Date: Thu, 2 May 2019 05:28:08 +0100
+Subject: [PATCH] x86/fpu: Export __kernel_fpu_{begin,end}()
+
+This partially undo commit:
+
+12209993  x86/fpu: Don't export __kernel_fpu_{begin,end}()
+
+We need this symbol in zfs for AES-NI/AVX support.
+---
+ arch/x86/include/asm/fpu/api.h | 2 ++
+ arch/x86/kernel/fpu/core.c     | 6 ++++--
+ 2 files changed, 6 insertions(+), 2 deletions(-)
+
+diff --git a/arch/x86/include/asm/fpu/api.h b/arch/x86/include/asm/fpu/api.h
+index b56d504af6545..7d53388d266ea 100644
+--- a/arch/x86/include/asm/fpu/api.h
++++ b/arch/x86/include/asm/fpu/api.h
+@@ -18,6 +18,8 @@
+  * If you intend to use the FPU in softirq you need to check first with
+  * irq_fpu_usable() if it is possible.
+  */
++extern void __kernel_fpu_begin(void);
++extern void __kernel_fpu_end(void);
+ extern void kernel_fpu_begin(void);
+ extern void kernel_fpu_end(void);
+ extern bool irq_fpu_usable(void);
+diff --git a/arch/x86/kernel/fpu/core.c b/arch/x86/kernel/fpu/core.c
+index 2e5003fef51a9..2ea85b32421a0 100644
+--- a/arch/x86/kernel/fpu/core.c
++++ b/arch/x86/kernel/fpu/core.c
+@@ -93,7 +93,7 @@ bool irq_fpu_usable(void)
+ }
+ EXPORT_SYMBOL(irq_fpu_usable);
+
+-static void __kernel_fpu_begin(void)
++void __kernel_fpu_begin(void)
+ {
+ 	struct fpu *fpu = &current->thread.fpu;
+
+@@ -111,8 +111,9 @@ static void __kernel_fpu_begin(void)
+ 		__cpu_invalidate_fpregs_state();
+ 	}
+ }
++EXPORT_SYMBOL(__kernel_fpu_begin);
+
+-static void __kernel_fpu_end(void)
++void __kernel_fpu_end(void)
+ {
+ 	struct fpu *fpu = &current->thread.fpu;
+
+@@ -121,6 +122,7 @@ static void __kernel_fpu_end(void)
+
+ 	kernel_fpu_enable();
+ }
++EXPORT_SYMBOL(__kernel_fpu_end);
+
+ void kernel_fpu_begin(void)
+ {

pkgs/os-specific/linux/kernel/patches.nix

@@ -57,4 +57,9 @@ rec {
       sha256 = "1l8xq02rd7vakxg52xm9g4zng0ald866rpgm8kjlh88mwwyjkrwv";
     };
   };
+
+  export_kernel_fpu_functions = rec {
+    name = "export_kernel_fpu_functions";
+    patch = ./export_kernel_fpu_functions.patch;
+  };
 }

pkgs/top-level/all-packages.nix

@@ -15061,6 +15061,7 @@ in
     kernelPatches =
       [ kernelPatches.bridge_stp_helper
         kernelPatches.modinst_arg_list_too_long
+        kernelPatches.export_kernel_fpu_functions
       ];
   };