kernel/x86_64: remove lfence after swapgs on syscall entry and exit

in a syscall, swapgs is always executed and can't be speculatively executed or bypassed.
it's also not needed on exception/interrupt exit, only on exception/interrupt entry.
follow-up on commit 84f6e2d39f by waddlesplash

(https://github.com/freebsd/freebsd-src/commit/7aa47cace14948a7b8277a4b24a0ca9e0308990a)

Change-Id: I56de9526a1acd0075c4a12147ae782f0366dec52
Reviewed-on: https://review.haiku-os.org/c/haiku/+/4557
Tested-by: Commit checker robot <[email protected]>
Reviewed-by: Fredrik Holmqvist <[email protected]>
Reviewed-by: Adrien Destugues <[email protected]>

src/system/kernel/arch/x86/64/entry_compat.S

@@ -152,7 +152,6 @@ FUNCTION_END(x86_64_syscall32_entry)
 //	ecx - user esp
 FUNCTION(x86_64_sysenter32_entry):
 	swapgs
-	lfence
 
 	// Set up an iframe on the stack (ECX = saved ESP).
 	push	$USER_DATA_SELECTOR			// ss
@@ -315,7 +314,6 @@ FUNCTION(x86_64_sysenter32_entry):
 
 	// Restore previous GS base and return.
 	swapgs
-	lfence
 	sti
 	sysexit
 

src/system/kernel/arch/x86/64/interrupts.S

@@ -328,7 +328,6 @@ STATIC_FUNCTION(int_bottom_user):
 
 	// Restore the previous GS base and return.
 	swapgs
-	lfence
 	iretq
 
 .Lkernel_exit_work:
@@ -362,7 +361,6 @@ STATIC_FUNCTION(int_bottom_user):
 
 	// Restore the previous GS base and return.
 	swapgs
-	lfence
 	iretq
 
 .Lkernel_exit_handle_signals:
@@ -382,7 +380,6 @@ FUNCTION(x86_64_syscall_entry):
 	// scratch space to store the user stack pointer in before we can push it
 	// to the stack.
 	swapgs
-	lfence
 	movq	%rsp, %gs:ARCH_THREAD_user_rsp
 	movq	%gs:ARCH_THREAD_syscall_rsp, %rsp
 
@@ -495,7 +492,6 @@ FUNCTION(x86_64_syscall_entry):
 
 	// Restore previous GS base and return.
 	swapgs
-	lfence
 	sysretq
 
 .Lpre_syscall_debug:
@@ -574,7 +570,6 @@ FUNCTION(x86_64_syscall_entry):
 
 	// Restore the previous GS base and return.
 	swapgs
-	lfence
 	iretq
 
 .Lpost_syscall_handle_signals:
@@ -651,7 +646,6 @@ FUNCTION(x86_return_to_userland):
 	// Restore the frame and return.
 	RESTORE_IFRAME()
 	swapgs
-	lfence
 	iretq
 .Luserland_return_work:
 	// Slow path for return to userland.
@@ -676,7 +670,6 @@ FUNCTION(x86_return_to_userland):
 
 	// Restore the previous GS base and return.
 	swapgs
-	lfence
 	iretq
 .Luserland_return_handle_signals:
 	// thread_at_kernel_exit requires interrupts to be enabled, it will disable