Merge tag 'x86-fred-2024-09-17' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip

Pull x86 FRED updates from Thomas Gleixner:

 - Enable FRED right after init_mem_mapping() because at that point the
   early IDT fault handler is replaced by the real fault handler. The
   real fault handler retrieves the faulting address from the stack
   frame and not from CR2 when the FRED feature is set. But that
   obviously only works when FRED is enabled in the CPU as well.

 - Set SS to __KERNEL_DS when enabling FRED to prevent a corner case
   where ERETS can observe a SS mismatch and raises a #GP.

* tag 'x86-fred-2024-09-17' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip:
  x86/entry: Set FRED RSP0 on return to userspace instead of context switch
  x86/msr: Switch between WRMSRNS and WRMSR with the alternatives mechanism
  x86/entry: Test ti_work for zero before processing individual bits
  x86/fred: Set SS to __KERNEL_DS when enabling FRED
  x86/fred: Enable FRED right after init_mem_mapping()
  x86/fred: Move FRED RSP initialization into a separate function
  x86/fred: Parse cmdline param "fred=" in cpu_parse_early_param()

1 files changed, 20 insertions, 2 deletions

diff --git a/arch/x86/kernel/cpu/common.c b/arch/x86/kernel/cpu/common.c
index be307c9ef263..07a34d723505 100644
--- a/arch/x86/kernel/cpu/common.c
+++ b/arch/x86/kernel/cpu/common.c
@@ -1510,6 +1510,11 @@ static void __init cpu_parse_early_param(void)
 	if (cmdline_find_option_bool(boot_command_line, "nousershstk"))
 		setup_clear_cpu_cap(X86_FEATURE_USER_SHSTK);
 
+	/* Minimize the gap between FRED is available and available but disabled. */
+	arglen = cmdline_find_option(boot_command_line, "fred", arg, sizeof(arg));
+	if (arglen != 2 || strncmp(arg, "on", 2))
+		setup_clear_cpu_cap(X86_FEATURE_FRED);
+
 	arglen = cmdline_find_option(boot_command_line, "clearcpuid", arg, sizeof(arg));
 	if (arglen <= 0)
 		return;
@@ -2171,7 +2176,7 @@ static inline void tss_setup_io_bitmap(struct tss_struct *tss)
  * Setup everything needed to handle exceptions from the IDT, including the IST
  * exceptions which use paranoid_entry().
  */
-void cpu_init_exception_handling(void)
+void cpu_init_exception_handling(bool boot_cpu)
 {
 	struct tss_struct *tss = this_cpu_ptr(&cpu_tss_rw);
 	int cpu = raw_smp_processor_id();
@@ -2190,10 +2195,23 @@ void cpu_init_exception_handling(void)
 	/* GHCB needs to be setup to handle #VC. */
 	setup_ghcb();
 
+	if (cpu_feature_enabled(X86_FEATURE_FRED)) {
+		/* The boot CPU has enabled FRED during early boot */
+		if (!boot_cpu)
+			cpu_init_fred_exceptions();
+
+		cpu_init_fred_rsps();
+	} else {
+		load_current_idt();
+	}
+}
+
+void __init cpu_init_replace_early_idt(void)
+{
 	if (cpu_feature_enabled(X86_FEATURE_FRED))
 		cpu_init_fred_exceptions();
 	else
-		load_current_idt();
+		idt_setup_early_pf();
 }
 
 /*