Re: [PATCH 07/29] x86/fpu: don't export __kernel_fpu_{begin|end}()

[Rik van Riel]

On Wed, 2018-11-28 at 23:20 +0100, Sebastian Andrzej Siewior wrote:
> 
> + * Use kernel_fpu_begin/end() if you intend to use FPU in kernel
> context. It
> + * disables preemption so be carefull if you intend to use it for
> long periods

Just how careful do you want to be?

One l seems sufficient :)

Feel free to add my Reviewed-by: once that spelling
error is fixed :)

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All Rights Reversed.


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[PATCH 07/29] x86/fpu: don't export __kernel_fpu_{begin|end}()

[Sebastian Andrzej Siewior]

There is one user using __kernel_fpu_begin() and before invoking it, it
invokes preempt_disable(). So it could invoke kernel_fpu_begin() right
away. The 32bit version of arch_efi_call_virt_setup() and
arch_efi_call_virt_teardown() does this already.

The comment above *kernel_fpu*() claims that before invoking
__kernel_fpu_begin() preemption should be disabled and that KVM is a
good example of doing it. Well, KVM doesn't do that since commit

  f775b13eedee2 ("x86,kvm: move qemu/guest FPU switching out to vcpu_run")

so it is not an example anymore.
With EFI gone as the last user of __kernel_fpu_{begin|end}(), it can be
made static and not exported anymore.

Signed-off-by: Sebastian Andrzej Siewior 
---
 arch/x86/include/asm/efi.h |  6 ++
 arch/x86/include/asm/fpu/api.h | 16 ++--
 arch/x86/kernel/fpu/core.c |  6 ++
 3 files changed, 10 insertions(+), 18 deletions(-)

diff --git a/arch/x86/include/asm/efi.h b/arch/x86/include/asm/efi.h
index eea40d52ca785..45864898f7e50 100644
--- a/arch/x86/include/asm/efi.h
+++ b/arch/x86/include/asm/efi.h
@@ -82,8 +82,7 @@ struct efi_scratch {
 #define arch_efi_call_virt_setup() \
 ({ \
efi_sync_low_kernel_mappings(); \
-   preempt_disable();  \
-   __kernel_fpu_begin();   \
+   kernel_fpu_begin(); \
firmware_restrict_branch_speculation_start();   \
\
if (!efi_enabled(EFI_OLD_MEMMAP))   \
@@ -99,8 +98,7 @@ struct efi_scratch {
efi_switch_mm(efi_scratch.prev_mm); \
\
firmware_restrict_branch_speculation_end(); \
-   __kernel_fpu_end(); \
-   preempt_enable();   \
+   kernel_fpu_end();   \
 })
 
 extern void __iomem *__init efi_ioremap(unsigned long addr, unsigned long size,
diff --git a/arch/x86/include/asm/fpu/api.h b/arch/x86/include/asm/fpu/api.h
index a9caac9d4a729..d6e094c282779 100644
--- a/arch/x86/include/asm/fpu/api.h
+++ b/arch/x86/include/asm/fpu/api.h
@@ -12,17 +12,13 @@
 #define _ASM_X86_FPU_API_H
 
 /*
- * Careful: __kernel_fpu_begin/end() must be called with preempt disabled
- * and they don't touch the preempt state on their own.
- * If you enable preemption after __kernel_fpu_begin(), preempt notifier
- * should call the __kernel_fpu_end() to prevent the kernel/user FPU
- * state from getting corrupted. KVM for example uses this model.
- *
- * All other cases use kernel_fpu_begin/end() which disable preemption
- * during kernel FPU usage.
+ * Use kernel_fpu_begin/end() if you intend to use FPU in kernel context. It
+ * disables preemption so be carefull if you intend to use it for long periods
+ * of time.
+ * If you intend to use the FPU in softirq you need to check first with
+ * irq_fpu_usable() if it is possible.
+ * Using the FPU in hardirq is not allowed.
  */
-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 2ea85b32421a0..2e5003fef51a9 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);
 
-void __kernel_fpu_begin(void)
+static void __kernel_fpu_begin(void)
 {
struct fpu *fpu = ¤t->thread.fpu;
 
@@ -111,9 +111,8 @@ void __kernel_fpu_begin(void)
__cpu_invalidate_fpregs_state();
}
 }
-EXPORT_SYMBOL(__kernel_fpu_begin);
 
-void __kernel_fpu_end(void)
+static void __kernel_fpu_end(void)
 {
struct fpu *fpu = ¤t->thread.fpu;
 
@@ -122,7 +121,6 @@ void __kernel_fpu_end(void)
 
kernel_fpu_enable();
 }
-EXPORT_SYMBOL(__kernel_fpu_end);
 
 void kernel_fpu_begin(void)
 {
-- 
2.20.0.rc1