[PATCH v1 1/1] x86/fred: Fix int80 emulation for FRED

["Xin Li (Intel)" <xin@zytor.com>]

Commit 55617fb991df added a bunch of tests to the int $0x80 path, however
they are unnecessary and event wrong in fact under FRED.

First FRED distinguishes external interrupts from software interrupts,
thus int80_emulation() should NEVER be called for handling an external
interrupt, and then int80_is_external() should be skipped under FRED.

Second, the FRED kernel entry handler NEVER dispatches INTx, which is
of event type EVENT_TYPE_SWINT, so the user mode checking in
do_int80_emulation() is redundant, and should be skipped.

It might be even better to strip down do_int80_emulation() to a lean
fred_int80_emulation(), not to mention int80_emulation() does a
CLEAR_BRANCH_HISTORY.

Suggested-by: H. Peter Anvin (Intel) <hpa@zytor.com>
Signed-off-by: Xin Li (Intel) <xin@zytor.com>
---
 arch/x86/entry/common.c | 22 ++++++++++++++++++++--
 1 file changed, 20 insertions(+), 2 deletions(-)

diff --git a/arch/x86/entry/common.c b/arch/x86/entry/common.c
index 6de50b80702e..aed745fc8333 100644
--- a/arch/x86/entry/common.c
+++ b/arch/x86/entry/common.c
@@ -174,6 +174,18 @@ static __always_inline bool int80_is_external(void)
 	const unsigned int offs = (0x80 / 32) * 0x10;
 	const u32 bit = BIT(0x80 % 32);
 
+	/*
+	 * FRED distinguishes external interrupts from software interrupts
+	 * with different event types, EVENT_TYPE_EXTINT v.s. EVENT_TYPE_SWINT,
+	 * thus we should NEVER get here when FRED is enabled, and then the
+	 * following APIC ISR check makes no sense.
+	 *
+	 * Furthermore, the external interrupt vector 0x80 is available as
+	 * a hardware interrupt under FRED.
+	 */
+	if (cpu_feature_enabled(X86_FEATURE_FRED))
+		return false;
+
 	/* The local APIC on XENPV guests is fake */
 	if (cpu_feature_enabled(X86_FEATURE_XENPV))
 		return false;
@@ -211,8 +223,14 @@ __visible noinstr void do_int80_emulation(struct pt_regs *regs)
 {
 	int nr;
 
-	/* Kernel does not use INT $0x80! */
-	if (unlikely(!user_mode(regs))) {
+	/*
+	 * Kernel does not use INT $0x80!
+	 *
+	 * The FRED kernel entry handler NEVER dispatches INTx (panic or
+	 * oops otherwise), so it is redundant to check user mode here.
+	 */
+	if (!cpu_feature_enabled(X86_FEATURE_FRED) &&
+	    unlikely(!user_mode(regs))) {
 		irqentry_enter(regs);
 		instrumentation_begin();
 		panic("Unexpected external interrupt 0x80\n");

base-commit: 86d1b22a75cffa50160e4621da00311e6f6f48de
-- 
2.44.0