[PATCH v2] x86/fpu: make WARN_ON_FPU get fully optimized out

[PATCH v2] x86/fpu: make WARN_ON_FPU get fully optimized out

[Eric Biggers]

From: Eric Biggers <ebiggers@google.com>

Currently WARN_ON_FPU evaluates its argument even if
CONFIG_X86_DEBUG_FPU is disabled, which adds unnecessary instructions to
several functions, for example kernel_fpu_begin().  Fix this by using
BUILD_BUG_ON_INVALID(x) in the no-debug case rather than (void)(x).

Fixes: 83242c515881 ("x86/fpu: Make WARN_ON_FPU() more robust in the !CONFIG_X86_DEBUG_FPU case")
Suggested-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Eric Biggers <ebiggers@google.com>
---

v2: use BUILD_BUG_ON_INVALID() as suggested by Sean Christopherson.

 arch/x86/kernel/fpu/internal.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/x86/kernel/fpu/internal.h b/arch/x86/kernel/fpu/internal.h
index dbdb31f55fc7f..975de070c9c98 100644
--- a/arch/x86/kernel/fpu/internal.h
+++ b/arch/x86/kernel/fpu/internal.h
@@ -16,11 +16,11 @@ static __always_inline __pure bool use_fxsr(void)
 }
 
 #ifdef CONFIG_X86_DEBUG_FPU
 # define WARN_ON_FPU(x) WARN_ON_ONCE(x)
 #else
-# define WARN_ON_FPU(x) ({ (void)(x); 0; })
+# define WARN_ON_FPU(x) ({ BUILD_BUG_ON_INVALID(x); 0; })
 #endif
 
 /* Used in init.c */
 extern void fpstate_init_user(struct fpstate *fpstate);
 extern void fpstate_reset(struct fpu *fpu);

base-commit: 805ba04cb7ccfc7d72e834ebd796e043142156ba
-- 
2.48.1

Re: [PATCH v2] x86/fpu: make WARN_ON_FPU get fully optimized out

[Eric Biggers]

On Mon, Jan 27, 2025 at 02:45:23PM -0800, Eric Biggers wrote:
> From: Eric Biggers <ebiggers@google.com>
> 
> Currently WARN_ON_FPU evaluates its argument even if
> CONFIG_X86_DEBUG_FPU is disabled, which adds unnecessary instructions to
> several functions, for example kernel_fpu_begin().  Fix this by using
> BUILD_BUG_ON_INVALID(x) in the no-debug case rather than (void)(x).
> 
> Fixes: 83242c515881 ("x86/fpu: Make WARN_ON_FPU() more robust in the !CONFIG_X86_DEBUG_FPU case")
> Suggested-by: Sean Christopherson <seanjc@google.com>
> Signed-off-by: Eric Biggers <ebiggers@google.com>
> ---
> 
> v2: use BUILD_BUG_ON_INVALID() as suggested by Sean Christopherson.
> 
>  arch/x86/kernel/fpu/internal.h | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)

Ping.  Any interest in taking this through the x86 tree?

- Eric