[RFC 1/2] x86/entry/64: Use the TSS sp2 slot for rsp_scratch

[Andy Lutomirski]

In the non-trampoline SYSCALL64 path, we use a percpu variable to
temporarily store the user RSP value.  Instead of a separate
variable, use the otherwise unused sp2 slot in the TSS.  This will
improve cache locality, as the sp1 slot is already used in the same
code to find the kernel stack.  It will also simplify a future
change to make the non-trampoline path work in PTI mode.

Signed-off-by: Andy Lutomirski <luto@xxxxxxxxxx>
---
 arch/x86/include/asm/processor.h   | 5 +++++
 arch/x86/include/asm/thread_info.h | 1 +
 arch/x86/kernel/asm-offsets_64.c   | 1 +
 arch/x86/kernel/process_64.c       | 2 --
 4 files changed, 7 insertions(+), 2 deletions(-)

diff --git a/arch/x86/include/asm/processor.h b/arch/x86/include/asm/processor.h
index cfd29ee8c3da..2ef4c39ded45 100644
--- a/arch/x86/include/asm/processor.h
+++ b/arch/x86/include/asm/processor.h
@@ -308,7 +308,12 @@ struct x86_hw_tss {
 	 */
 	u64			sp1;
 
+	/*
+	 * sp2 is scratch space used by the SYSCALL64 handler.  Linux does
+	 * not use rung 2, so sp2 is not otherwise needed.
+	 */
 	u64			sp2;
+
 	u64			reserved2;
 	u64			ist[7];
 	u32			reserved3;
diff --git a/arch/x86/include/asm/thread_info.h b/arch/x86/include/asm/thread_info.h
index 2ff2a30a264f..9a2f84233e39 100644
--- a/arch/x86/include/asm/thread_info.h
+++ b/arch/x86/include/asm/thread_info.h
@@ -209,6 +209,7 @@ static inline int arch_within_stack_frames(const void * const stack,
 
 #ifdef CONFIG_X86_64
 # define cpu_current_top_of_stack (cpu_tss_rw + TSS_sp1)
+# define rsp_scratch (cpu_tss_rw + TSS_sp2)
 #endif
 
 #endif
diff --git a/arch/x86/kernel/asm-offsets_64.c b/arch/x86/kernel/asm-offsets_64.c
index b2dcd161f514..621bf6b5a63b 100644
--- a/arch/x86/kernel/asm-offsets_64.c
+++ b/arch/x86/kernel/asm-offsets_64.c
@@ -67,6 +67,7 @@ int main(void)
 	OFFSET(TSS_ist, tss_struct, x86_tss.ist);
 	OFFSET(TSS_sp0, tss_struct, x86_tss.sp0);
 	OFFSET(TSS_sp1, tss_struct, x86_tss.sp1);
+	OFFSET(TSS_sp2, tss_struct, x86_tss.sp2);
 	BLANK();
 
 #ifdef CONFIG_STACKPROTECTOR
diff --git a/arch/x86/kernel/process_64.c b/arch/x86/kernel/process_64.c
index 12bb445fb98d..3ed5fed181cc 100644
--- a/arch/x86/kernel/process_64.c
+++ b/arch/x86/kernel/process_64.c
@@ -59,8 +59,6 @@
 #include <asm/unistd_32_ia32.h>
 #endif
 
-__visible DEFINE_PER_CPU(unsigned long, rsp_scratch);
-
 /* Prints also some state that isn't saved in the pt_regs */
 void __show_regs(struct pt_regs *regs, int all)
 {
-- 
2.17.1