linux-kernel - [PATCH v2] x86/fpu: make WARN_ON

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Message-ID: <20250127224523.94300-1-ebiggers@kernel.org>
Date: Mon, 27 Jan 2025 14:45:23 -0800
From: Eric Biggers <ebiggers@...nel.org>
To: x86@...nel.org
Cc: linux-kernel@...r.kernel.org,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	Borislav Petkov <bp@...en8.de>,
	Dave Hansen <dave.hansen@...ux.intel.com>,
	"H . Peter Anvin" <hpa@...or.com>,
	Sean Christopherson <seanjc@...gle.com>
Subject: [PATCH v2] x86/fpu: make WARN_ON_FPU get fully optimized out

From: Eric Biggers <ebiggers@...gle.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@...gle.com>
Signed-off-by: Eric Biggers <ebiggers@...gle.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