linux-kernel - [PATCH 07/14] arm64: ssbd: Skip apply_ssbd if not using dynamic mitigation

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Date:   Tue, 22 May 2018 16:06:41 +0100
From:   Marc Zyngier <marc.zyngier@....com>
To:     linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        kvmarm@...ts.cs.columbia.edu
Cc:     Will Deacon <will.deacon@....com>,
        Catalin Marinas <catalin.marinas@....com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Andy Lutomirski <luto@...nel.org>,
        Kees Cook <keescook@...omium.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Christoffer Dall <christoffer.dall@....com>
Subject: [PATCH 07/14] arm64: ssbd: Skip apply_ssbd if not using dynamic mitigation

In order to avoid checking arm64_ssbd_callback_required on each
kernel entry/exit even if no mitigation is required, let's
add yet another alternative that by default jumps over the mitigation,
and that gets nop'ed out if we're doing dynamic mitigation.

Think of it as a poor man's static key...

Signed-off-by: Marc Zyngier <marc.zyngier@....com>
---
 arch/arm64/kernel/cpu_errata.c | 14 ++++++++++++++
 arch/arm64/kernel/entry.S      |  3 +++
 2 files changed, 17 insertions(+)

diff --git a/arch/arm64/kernel/cpu_errata.c b/arch/arm64/kernel/cpu_errata.c
index f1d4e75b0ddd..8f686f39b9c1 100644
--- a/arch/arm64/kernel/cpu_errata.c
+++ b/arch/arm64/kernel/cpu_errata.c
@@ -283,6 +283,20 @@ void __init arm64_update_smccc_conduit(struct alt_instr *alt,
 	*updptr = cpu_to_le32(insn);
 }
 
+void __init arm64_enable_wa2_handling(struct alt_instr *alt,
+				      __le32 *origptr, __le32 *updptr,
+				      int nr_inst)
+{
+	BUG_ON(nr_inst != 1);
+	/*
+	 * Only allow mitigation on EL1 entry/exit and guest
+	 * ARCH_WORKAROUND_2 handling if the SSBD state allows it to
+	 * be flipped.
+	 */
+	if (arm64_get_ssbd_state() == ARM64_SSBD_EL1_ENTRY)
+		*updptr = cpu_to_le32(aarch64_insn_gen_nop());
+}
+
 static void do_ssbd(bool state)
 {
 	switch (psci_ops.conduit) {
diff --git a/arch/arm64/kernel/entry.S b/arch/arm64/kernel/entry.S
index 29ad672a6abd..e6f6e2339b22 100644
--- a/arch/arm64/kernel/entry.S
+++ b/arch/arm64/kernel/entry.S
@@ -142,6 +142,9 @@ alternative_else_nop_endif
 	// to save/restore them if required.
 	.macro	apply_ssbd, state, targ, tmp1, tmp2
 #ifdef CONFIG_ARM64_SSBD
+alternative_cb	arm64_enable_wa2_handling
+	b	\targ
+alternative_cb_end
 	ldr_this_cpu	\tmp2, arm64_ssbd_callback_required, \tmp1
 	cbz	\tmp2, \targ
 	mov	w0, #ARM_SMCCC_ARCH_WORKAROUND_2
-- 
2.14.2