[tip: x86/fred] x86/syscall: Split IDT syscall setup code into idt_syscall_init()

[tip-bot2 for Xin Li]

The following commit has been merged into the x86/fred branch of tip:

Commit-ID:     ae46f3978ae4eb9da013ff9963f105de2db2f8ec
Gitweb:        https://git.kernel.org/tip/ae46f3978ae4eb9da013ff9963f105de2db2f8ec
Author:        Xin Li <xin3.li@xxxxxxxxx>
AuthorDate:    Tue, 05 Dec 2023 02:50:22 -08:00
Committer:     Borislav Petkov (AMD) <bp@xxxxxxxxx>
CommitterDate: Tue, 30 Jan 2024 18:20:36 +01:00

x86/syscall: Split IDT syscall setup code into idt_syscall_init()

Because FRED uses the ring 3 FRED entrypoint for SYSCALL and SYSENTER and
ERETU is the only legit instruction to return to ring 3, there is NO need
to setup SYSCALL and SYSENTER MSRs for FRED, except the IA32_STAR MSR.

Split IDT syscall setup code into idt_syscall_init() to make it easy to
skip syscall setup code when FRED is enabled.

Suggested-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Signed-off-by: Xin Li <xin3.li@xxxxxxxxx>
Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Tested-by: Shan Kang <shan.kang@xxxxxxxxx>
Link: https://lore.kernel.org/r/20231205105030.8698-34-xin3.li@xxxxxxxxx

---
 arch/x86/kernel/cpu/common.c | 13 ++++++++++---
 1 file changed, 10 insertions(+), 3 deletions(-)

diff --git a/arch/x86/kernel/cpu/common.c b/arch/x86/kernel/cpu/common.c
index c3a1757..4f5e4aa 100644
--- a/arch/x86/kernel/cpu/common.c
+++ b/arch/x86/kernel/cpu/common.c
@@ -2066,10 +2066,8 @@ static void wrmsrl_cstar(unsigned long val)
 		wrmsrl(MSR_CSTAR, val);
 }
 
-/* May not be marked __init: used by software suspend */
-void syscall_init(void)
+static inline void idt_syscall_init(void)
 {
-	wrmsr(MSR_STAR, 0, (__USER32_CS << 16) | __KERNEL_CS);
 	wrmsrl(MSR_LSTAR, (unsigned long)entry_SYSCALL_64);
 
 	if (ia32_enabled()) {
@@ -2103,6 +2101,15 @@ void syscall_init(void)
 	       X86_EFLAGS_AC|X86_EFLAGS_ID);
 }
 
+/* May not be marked __init: used by software suspend */
+void syscall_init(void)
+{
+	/* The default user and kernel segments */
+	wrmsr(MSR_STAR, 0, (__USER32_CS << 16) | __KERNEL_CS);
+
+	idt_syscall_init();
+}
+
 #else	/* CONFIG_X86_64 */
 
 #ifdef CONFIG_STACKPROTECTOR