kernel/x86_64: clear any pending exceptions on #MF

* also on x86 for simplicity.
* fixes #18624
* also makes x87 FPU data registers available for x87 floating instructions. EMMS is cheap.
see https://github.com/cloudius-systems/osv/commit/25209d81f7b872111beb02ab9758f0d86898ec6b

Change-Id: I5c1b399377102f3eb10bc6d7f7247afbaf6d8483
Reviewed-on: https://review.haiku-os.org/c/haiku/+/7089
Reviewed-by: waddlesplash <[email protected]>
Reviewed-by: Adrien Destugues <[email protected]>

headers/private/kernel/arch/x86/64/cpu.h

@@ -48,6 +48,8 @@ x86_context_switch(arch_thread* oldState, arch_thread* newState)
 			"r14", "r15", "xmm0", "xmm1", "xmm2", "xmm3", "xmm4", "xmm5",
 			"xmm6", "xmm7", "xmm8", "xmm9", "xmm10", "xmm11", "xmm12", "xmm13",
 			"xmm14", "xmm15", "memory");
+	// so that x87 FPU floating-point instructions can be executed
+	asm volatile("emms");
 	asm volatile("ldmxcsr %0" : : "m" (sseControl));
 	asm volatile("fldcw %0" : : "m" (fpuControl));
 }

src/system/kernel/arch/x86/arch_int.cpp

@@ -161,6 +161,9 @@ x86_unexpected_exception(iframe* frame)
 				// TODO: Determine the correct cause via the FPU status
 				// register!
 			signalAddress = frame->ip;
+			// clear any pending exceptions, otherwise loading a new control word
+			// could raise exceptions.
+			asm volatile("fnclex");
 			break;
 
 		case 17: 	// Alignment Check Exception (#AC)