[RFC PATCH 19/32] x86/fred: add a NMI entry stub for FRED

[Xin Li <xin3.li@intel.com>]

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

On a FRED system, NMIs nest both with themselves and faults, transient
information is saved into the stack frame, and NMI unblocking only
happens when the stack frame indicates that so should happen.

Thus, the NMI entry stub for FRED is really quite small...

Signed-off-by: H. Peter Anvin (Intel) <hpa@zytor.com>
Signed-off-by: Xin Li <xin3.li@intel.com>
---
 arch/x86/include/asm/fred.h |  1 +
 arch/x86/kernel/nmi.c       | 28 ++++++++++++++++++++++++++++
 2 files changed, 29 insertions(+)

diff --git a/arch/x86/include/asm/fred.h b/arch/x86/include/asm/fred.h
index 3089d1c70771..66c274a12e26 100644
--- a/arch/x86/include/asm/fred.h
+++ b/arch/x86/include/asm/fred.h
@@ -92,6 +92,7 @@ static __always_inline unsigned long fred_event_data(struct pt_regs *regs)
 #define DEFINE_FRED_HANDLER(f) noinstr DECLARE_FRED_HANDLER(f)
 typedef DECLARE_FRED_HANDLER((*fred_handler));
 
+DECLARE_FRED_HANDLER(fred_exc_nmi);
 DECLARE_FRED_HANDLER(fred_exc_debug);
 DECLARE_FRED_HANDLER(fred_exc_page_fault);
 
diff --git a/arch/x86/kernel/nmi.c b/arch/x86/kernel/nmi.c
index cec0bfa3bc04..d497071a79f2 100644
--- a/arch/x86/kernel/nmi.c
+++ b/arch/x86/kernel/nmi.c
@@ -34,6 +34,7 @@
 #include <asm/cache.h>
 #include <asm/nospec-branch.h>
 #include <asm/sev.h>
+#include <asm/fred.h>
 
 #define CREATE_TRACE_POINTS
 #include <trace/events/nmi.h>
@@ -537,6 +538,33 @@ DEFINE_IDTENTRY_RAW(exc_nmi_noist)
 EXPORT_SYMBOL_GPL(asm_exc_nmi_noist);
 #endif
 
+#ifdef CONFIG_X86_FRED
+DEFINE_FRED_HANDLER(fred_exc_nmi)
+{
+	/*
+	 * With FRED, CR2 and DR6 are pushed atomically on faults,
+	 * so we don't have to worry about saving and restoring them.
+	 * Breakpoint faults nest, so assume it is OK to leave DR7
+	 * enabled.
+	 */
+	irqentry_state_t irq_state = irqentry_nmi_enter(regs);
+
+	/*
+	 * VM exit induced by a NMI keeps NMI blocked, and we do
+	 * "int $2" to reinject the NMI w/ NMI kept being blocked.
+	 * However "int $2" doesn't set the nmi bit in the FRED
+	 * stack frame, so we explicitly set it to make sure a
+	 * later ERETS will unblock NMI immediately.
+	 */
+	regs->nmi = 1;
+
+	inc_irq_stat(__nmi_count);
+	default_do_nmi(regs);
+
+	irqentry_nmi_exit(regs, irq_state);
+}
+#endif
+
 void stop_nmi(void)
 {
 	ignore_nmis++;
-- 
2.34.1