linux-kernel - [PATCH v2 5/5] x86/pti: Do not enable PTI on fixed Intel processors

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Message-Id: <1516726375-25168-6-git-send-email-dwmw@amazon.co.uk>
Date:   Tue, 23 Jan 2018 16:52:55 +0000
From:   David Woodhouse <dwmw@...zon.co.uk>
To:     arjan@...ux.intel.com, tglx@...utronix.de, karahmed@...zon.de,
        x86@...nel.org, linux-kernel@...r.kernel.org,
        tim.c.chen@...ux.intel.com, bp@...en8.de, peterz@...radead.org,
        pbonzini@...hat.com, ak@...ux.intel.com,
        torvalds@...ux-foundation.org, gregkh@...ux-foundation.org,
        thomas.lendacky@....com
Subject: [PATCH v2 5/5] x86/pti: Do not enable PTI on fixed Intel processors

When they advertise the IA32_ARCH_CAPABILITIES MSR and it has the RDCL_NO
bit set, they don't need KPTI either.

Signed-off-by: David Woodhouse <dwmw@...zon.co.uk>
---
 arch/x86/kernel/cpu/common.c | 10 ++++++++--
 1 file changed, 8 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kernel/cpu/common.c b/arch/x86/kernel/cpu/common.c
index e5d66e9..c05d0fe 100644
--- a/arch/x86/kernel/cpu/common.c
+++ b/arch/x86/kernel/cpu/common.c
@@ -900,8 +900,14 @@ static void __init early_identify_cpu(struct cpuinfo_x86 *c)
 
 	setup_force_cpu_cap(X86_FEATURE_ALWAYS);
 
-	if (c->x86_vendor != X86_VENDOR_AMD)
-		setup_force_cpu_bug(X86_BUG_CPU_MELTDOWN);
+	if (c->x86_vendor != X86_VENDOR_AMD) {
+		u64 ia32_cap = 0;
+
+		if (cpu_has(c, X86_FEATURE_ARCH_CAPABILITIES))
+			rdmsrl(MSR_IA32_ARCH_CAPABILITIES, ia32_cap);
+		if (!(ia32_cap & ARCH_CAP_RDCL_NO))
+			setup_force_cpu_bug(X86_BUG_CPU_MELTDOWN);
+	}
 
 	setup_force_cpu_bug(X86_BUG_SPECTRE_V1);
 	setup_force_cpu_bug(X86_BUG_SPECTRE_V2);
-- 
2.7.4