[RFC PATCH 27/32] x86/fred: allow single-step trap and NMI when starting a new thread

[Xin Li]

From: "H. Peter Anvin (Intel)" <hpa@xxxxxxxxx>

Allow single-step trap and NMI when starting a new thread, thus once
the new thread returns to ring3, single-step trap and NMI are both
enabled immediately.

High-order 48 bits above the lowest 16 bit CS are discarded by the
legacy IRET instruction, thus can be set unconditionally, even when
FRED is not enabled.

Signed-off-by: H. Peter Anvin (Intel) <hpa@xxxxxxxxx>
Signed-off-by: Xin Li <xin3.li@xxxxxxxxx>
---
 arch/x86/include/asm/fred.h  | 11 +++++++++++
 arch/x86/kernel/process_64.c | 13 +++++++------
 2 files changed, 18 insertions(+), 6 deletions(-)

diff --git a/arch/x86/include/asm/fred.h b/arch/x86/include/asm/fred.h
index b6308e351e14..730e69d2bb87 100644
--- a/arch/x86/include/asm/fred.h
+++ b/arch/x86/include/asm/fred.h
@@ -50,6 +50,14 @@
 #define FRED_CSL_ALLOW_SINGLE_STEP	_BITUL(25)
 #define FRED_CSL_INTERRUPT_SHADOW	_BITUL(24)
 
+/*
+ * High-order 48 bits above the lowest 16 bit CS are discarded by the
+ * legacy IRET instruction, thus can be set unconditionally, even when
+ * FRED is not enabled.
+ */
+#define CSL_PROCESS_START \
+	(FRED_CSL_ENABLE_NMI | FRED_CSL_ALLOW_SINGLE_STEP)
+
 #ifndef __ASSEMBLY__
 
 #include <linux/kernel.h>
@@ -113,6 +121,9 @@ void fred_setup_apic(void);
 #else
 #define cpu_init_fred_exceptions() BUG()
 #define fred_setup_apic() BUG()
+
+#define CSL_PROCESS_START 0
+
 #endif /* CONFIG_X86_FRED */
 
 #endif /* ASM_X86_FRED_H */
diff --git a/arch/x86/kernel/process_64.c b/arch/x86/kernel/process_64.c
index 5b6cfd2ca630..128dafc04acf 100644
--- a/arch/x86/kernel/process_64.c
+++ b/arch/x86/kernel/process_64.c
@@ -55,6 +55,7 @@
 #include <asm/resctrl.h>
 #include <asm/unistd.h>
 #include <asm/fsgsbase.h>
+#include <asm/fred.h>
 #ifdef CONFIG_IA32_EMULATION
 /* Not included via unistd.h */
 #include <asm/unistd_32_ia32.h>
@@ -506,7 +507,7 @@ void x86_gsbase_write_task(struct task_struct *task, unsigned long gsbase)
 static void
 start_thread_common(struct pt_regs *regs, unsigned long new_ip,
 		    unsigned long new_sp,
-		    unsigned int _cs, unsigned int _ss, unsigned int _ds)
+		    u16 _cs, u16 _ss, u16 _ds)
 {
 	WARN_ON_ONCE(regs != current_pt_regs());
 
@@ -521,11 +522,11 @@ start_thread_common(struct pt_regs *regs, unsigned long new_ip,
 	loadsegment(ds, _ds);
 	load_gs_index(0);
 
-	regs->ip		= new_ip;
-	regs->sp		= new_sp;
-	regs->cs		= _cs;
-	regs->ss		= _ss;
-	regs->flags		= X86_EFLAGS_IF;
+	regs->ip    = new_ip;
+	regs->sp    = new_sp;
+	regs->csl   = _cs | CSL_PROCESS_START;
+	regs->ssl   = _ss;
+	regs->flags = X86_EFLAGS_IF | X86_EFLAGS_FIXED;
 }
 
 void
-- 
2.34.1