[RFC PATCH 2/5] entry: Add calls for save/restore auxiliary pt_regs

[ira.weiny@intel.com]

From: Ira Weiny <ira.weiny@intel.com>

Some architectures have auxiliary pt_regs space available to store
information on the stack during exceptions.  This information is easier
to obtain and store within C code rather than in arch specific assembly.

Define empty calls to architecture specific save and restore auxiliary
pt_regs functions.  Call these functions on generic entry/exit.

NOTE: Due to the split nature of the Xen exit code
irqentry_exit_cond_resched() requires an unbalanced call to
arch_restore_aux_pt_regs().

Cc: Rik van Riel <riel@surriel.com>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: Borislav Petkov <bp@alien8.de>
Signed-off-by: Ira Weiny <ira.weiny@intel.com>

---
Forward ported from PKS series
	https://lore.kernel.org/lkml/20220419170649.1022246-20-ira.weiny@intel.com/
---
 include/linux/entry-common.h |  8 ++++++++
 kernel/entry/common.c        | 16 ++++++++++++++--
 2 files changed, 22 insertions(+), 2 deletions(-)

diff --git a/include/linux/entry-common.h b/include/linux/entry-common.h
index 976cce7cf803..1c09ba64ad28 100644
--- a/include/linux/entry-common.h
+++ b/include/linux/entry-common.h
@@ -79,6 +79,14 @@ static __always_inline void arch_enter_from_user_mode(struct pt_regs *regs);
 static __always_inline void arch_enter_from_user_mode(struct pt_regs *regs) {}
 #endif
 
+#ifndef arch_save_aux_pt_regs
+static inline void arch_save_aux_pt_regs(struct pt_regs *regs) { }
+#endif
+
+#ifndef arch_restore_aux_pt_regs
+static inline void arch_restore_aux_pt_regs(struct pt_regs *regs) { }
+#endif
+
 /**
  * enter_from_user_mode - Establish state when coming from user mode
  *
diff --git a/kernel/entry/common.c b/kernel/entry/common.c
index 8c0f334c4b75..a70a0f314aee 100644
--- a/kernel/entry/common.c
+++ b/kernel/entry/common.c
@@ -317,7 +317,7 @@ noinstr irqentry_state_t irqentry_enter(struct pt_regs *regs)
 
 	if (user_mode(regs)) {
 		irqentry_enter_from_user_mode(regs);
-		return ret;
+		goto aux_save;
 	}
 
 	/*
@@ -356,7 +356,7 @@ noinstr irqentry_state_t irqentry_enter(struct pt_regs *regs)
 		instrumentation_end();
 
 		ret.exit_rcu = true;
-		return ret;
+		goto aux_save;
 	}
 
 	/*
@@ -371,6 +371,11 @@ noinstr irqentry_state_t irqentry_enter(struct pt_regs *regs)
 	trace_hardirqs_off_finish();
 	instrumentation_end();
 
+aux_save:
+	instrumentation_begin();
+	arch_save_aux_pt_regs(regs);
+	instrumentation_end();
+
 	return ret;
 }
 
@@ -401,6 +406,7 @@ void dynamic_irqentry_exit_cond_resched(void)
 
 void irqentry_exit_cond_resched(struct pt_regs *regs)
 {
+	arch_restore_aux_pt_regs(regs);
 	irqentry_exit_cond_resched_internal();
 }
 
@@ -408,6 +414,10 @@ noinstr void irqentry_exit(struct pt_regs *regs, irqentry_state_t state)
 {
 	lockdep_assert_irqs_disabled();
 
+	instrumentation_begin();
+	arch_restore_aux_pt_regs(regs);
+	instrumentation_end();
+
 	/* Check whether this returns to user mode */
 	if (user_mode(regs)) {
 		irqentry_exit_to_user_mode(regs);
@@ -459,6 +469,7 @@ irqentry_state_t noinstr irqentry_nmi_enter(struct pt_regs *regs)
 	instrumentation_begin();
 	trace_hardirqs_off_finish();
 	ftrace_nmi_enter();
+	arch_save_aux_pt_regs(regs);
 	instrumentation_end();
 
 	return irq_state;
@@ -467,6 +478,7 @@ irqentry_state_t noinstr irqentry_nmi_enter(struct pt_regs *regs)
 void noinstr irqentry_nmi_exit(struct pt_regs *regs, irqentry_state_t irq_state)
 {
 	instrumentation_begin();
+	arch_restore_aux_pt_regs(regs);
 	ftrace_nmi_exit();
 	if (irq_state.lockdep) {
 		trace_hardirqs_on_prepare();
-- 
2.35.3