[PATCH 4/4] x86/traps: Introduce early_traps_init() and simplify setup

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

Something I overlooked when last cleaning up exception handling is that a TSS
is not necessary if IST isn't configured, and IST isn't necessary until we're
running guest code.

Introduce early_traps_init() which is far more minimal than init_idt_traps();
bsp_ist[] is constructed without IST settings, so all early_traps_init() needs
to do is load the IDT, and invalidate TR/LDTR.

Put the remaining logic into traps_init(), later on boot.  Note that
load_system_tables() already contains enable_each_ist(), so the call is simply
dropped.

This removes some complexity prior to having exception support, and lays the
groundwork to not even allocate a TSS when using FRED.

No practical change.

Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
---
CC: Jan Beulich <JBeulich@xxxxxxxx>
CC: Roger Pau Monné <roger.pau@xxxxxxxxxx>
---
 xen/arch/x86/include/asm/system.h |  1 -
 xen/arch/x86/include/asm/traps.h  |  1 +
 xen/arch/x86/setup.c              |  3 +--
 xen/arch/x86/traps-setup.c        | 32 +++++++++++++++++++++++++++++++
 xen/arch/x86/traps.c              | 13 -------------
 5 files changed, 34 insertions(+), 16 deletions(-)

diff --git a/xen/arch/x86/include/asm/system.h 
b/xen/arch/x86/include/asm/system.h
index 1bf6bd026852..8ceaaf45d1a0 100644
--- a/xen/arch/x86/include/asm/system.h
+++ b/xen/arch/x86/include/asm/system.h
@@ -263,7 +263,6 @@ static inline int local_irq_is_enabled(void)
 #define BROKEN_ACPI_Sx          0x0001
 #define BROKEN_INIT_AFTER_S1    0x0002
 
-void init_idt_traps(void);
 void load_system_tables(void);
 void subarch_percpu_traps_init(void);
 
diff --git a/xen/arch/x86/include/asm/traps.h b/xen/arch/x86/include/asm/traps.h
index 3d30aa6738d4..72c33a33e283 100644
--- a/xen/arch/x86/include/asm/traps.h
+++ b/xen/arch/x86/include/asm/traps.h
@@ -7,6 +7,7 @@
 #ifndef ASM_TRAP_H
 #define ASM_TRAP_H
 
+void early_traps_init(void);
 void traps_init(void);
 void percpu_traps_init(void);
 
diff --git a/xen/arch/x86/setup.c b/xen/arch/x86/setup.c
index 5e2411a008f5..6a6ddc569500 100644
--- a/xen/arch/x86/setup.c
+++ b/xen/arch/x86/setup.c
@@ -1093,8 +1093,7 @@ void asmlinkage __init noreturn __start_xen(void)
 
     percpu_init_areas();
 
-    init_idt_traps();
-    load_system_tables();
+    early_traps_init();
 
     smp_prepare_boot_cpu();
     sort_exception_tables();
diff --git a/xen/arch/x86/traps-setup.c b/xen/arch/x86/traps-setup.c
index 1a7b42c14bf2..67a6fda89ad9 100644
--- a/xen/arch/x86/traps-setup.c
+++ b/xen/arch/x86/traps-setup.c
@@ -55,6 +55,31 @@ static void __init init_ler(void)
     setup_force_cpu_cap(X86_FEATURE_XEN_LBR);
 }
 
+/*
+ * Configure basic exception handling.  This is prior to parsing the command
+ * line or configuring a console, and needs to be as simple as possible.
+ *
+ * boot_gdt is already loaded, and bsp_idt[] is constructed without IST
+ * settings, so we don't need a TSS configured yet.
+ *
+ * Load bsp_idt[], and invalidate the TSS and LDT.
+ */
+void __init early_traps_init(void)
+{
+    const struct desc_ptr idtr = {
+        .base = (unsigned long)bsp_idt,
+        .limit = sizeof(bsp_idt) - 1,
+    };
+
+    lidt(&idtr);
+
+    _set_tssldt_desc(boot_gdt + TSS_ENTRY - FIRST_RESERVED_GDT_ENTRY,
+                     0, 0, SYS_DESC_tss_avail);
+
+    ltr(TSS_SELECTOR);
+    lldt(0);
+}
+
 /*
  * Configure complete exception, interrupt and syscall handling.
  */
@@ -63,6 +88,13 @@ void __init traps_init(void)
     /* Replace early pagefault with real pagefault handler. */
     _update_gate_addr_lower(&bsp_idt[X86_EXC_PF], entry_PF);
 
+    this_cpu(idt) = bsp_idt;
+    this_cpu(gdt) = boot_gdt;
+    if ( IS_ENABLED(CONFIG_PV32) )
+        this_cpu(compat_gdt) = boot_compat_gdt;
+
+    load_system_tables();
+
     init_ler();
 
     /* Cache {,compat_}gdt_l1e now that physically relocation is done. */
diff --git a/xen/arch/x86/traps.c b/xen/arch/x86/traps.c
index 79d92f21acf5..25e0d5777e6e 100644
--- a/xen/arch/x86/traps.c
+++ b/xen/arch/x86/traps.c
@@ -1858,19 +1858,6 @@ void asmlinkage do_entry_CP(struct cpu_user_regs *regs)
     panic("CONTROL-FLOW PROTECTION FAULT: #CP[%04x] %s\n", ec, err);
 }
 
-void __init init_idt_traps(void)
-{
-    /* Specify dedicated interrupt stacks for NMI, #DF, and #MC. */
-    enable_each_ist(bsp_idt);
-
-    /* CPU0 uses the master IDT. */
-    this_cpu(idt) = bsp_idt;
-
-    this_cpu(gdt) = boot_gdt;
-    if ( IS_ENABLED(CONFIG_PV32) )
-        this_cpu(compat_gdt) = boot_compat_gdt;
-}
-
 void asm_domain_crash_synchronous(unsigned long addr)
 {
     /*
-- 
2.39.5