x86/entry/64: Remove obsolete comment on tracing vs. SYSRET

This comment comes from a time when the kernel attempted to use SYSRET
on all returns to userspace, including interrupts and exceptions.  Ever
since commit fffbb5dc ("Move opportunistic sysret code to syscall code
path"), SYSRET is only used for returning from system calls. The
specific tracing issue listed in this comment is not possible anymore.

Signed-off-by: Brian Gerst <brgerst@gmail.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Brian Gerst <brgerst@gmail.com>
Cc: Denys Vlasenko <dvlasenk@redhat.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Josh Poimboeuf <jpoimboe@redhat.com>
Link: https://lore.kernel.org/r/20230721161018.50214-2-brgerst@gmail.com

diff --git a/arch/x86/entry/entry_64.S b/arch/x86/entry/entry_64.S
index f71664dea1a3b33e37ae4173b5afb5b66a15421b..7574639cb0c4ebfcccf4949c7d7fe40f65058faa 100644 (file)
--- a/arch/x86/entry/entry_64.S
+++ b/arch/x86/entry/entry_64.S
@@ -166,22 +166,9 @@ SYM_INNER_LABEL(entry_SYSCALL_64_after_hwframe, SYM_L_GLOBAL)
        jne     swapgs_restore_regs_and_return_to_usermode
 
        /*
-        * SYSCALL clears RF when it saves RFLAGS in R11 and SYSRET cannot
-        * restore RF properly. If the slowpath sets it for whatever reason, we
-        * need to restore it correctly.
-        *
-        * SYSRET can restore TF, but unlike IRET, restoring TF results in a
-        * trap from userspace immediately after SYSRET.  This would cause an
-        * infinite loop whenever #DB happens with register state that satisfies
-        * the opportunistic SYSRET conditions.  For example, single-stepping
-        * this user code:
-        *
-        *           movq       $stuck_here, %rcx
-        *           pushfq
-        *           popq %r11
-        *   stuck_here:
-        *
-        * would never get past 'stuck_here'.
+        * SYSRET cannot restore RF.  It can restore TF, but unlike IRET,
+        * restoring TF results in a trap from userspace immediately after
+        * SYSRET.
         */
        testq   $(X86_EFLAGS_RF|X86_EFLAGS_TF), %r11
        jnz     swapgs_restore_regs_and_return_to_usermode