x86/bugs: Add AMD's SPEC_CTRL MSR usage

commit 6ac2f49e upstream.

The AMD document outlining the SSBD handling
124441_AMD64_SpeculativeStoreBypassDisable_Whitepaper_final.pdf
mentions that if CPUID 8000_0008.EBX[24] is set we should be using
the SPEC_CTRL MSR (0x48) over the VIRT SPEC_CTRL MSR (0xC001_011f)
for speculative store bypass disable.

This in effect means we should clear the X86_FEATURE_VIRT_SSBD
flag so that we would prefer the SPEC_CTRL MSR.

See the document titled:
   124441_AMD64_SpeculativeStoreBypassDisable_Whitepaper_final.pdf

A copy of this document is available at
   https://bugzilla.kernel.org/show_bug.cgi?id=199889Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Tom Lendacky <thomas.lendacky@amd.com>
Cc: Janakarajan Natarajan <Janakarajan.Natarajan@amd.com>
Cc: kvm@vger.kernel.org
Cc: KarimAllah Ahmed <karahmed@amazon.de>
Cc: andrew.cooper3@citrix.com
Cc: Joerg Roedel <joro@8bytes.org>
Cc: Radim Krčmář <rkrcmar@redhat.com>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Borislav Petkov <bp@suse.de>
Cc: David Woodhouse <dwmw@amazon.co.uk>
Cc: Kees Cook <keescook@chromium.org>
Link: https://lkml.kernel.org/r/20180601145921.9500-3-konrad.wilk@oracle.com
[bwh: Backported to 4.9:
 - Update feature test in guest_cpuid_has_spec_ctrl() instead of
   svm_{get,set}_msr()
 - Adjust context, indentation]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

arch/x86/include/asm/cpufeatures.h

@@ -274,6 +274,7 @@
 #define X86_FEATURE_AMD_IBPB	(13*32+12) /* "" Indirect Branch Prediction Barrier */
 #define X86_FEATURE_AMD_IBRS	(13*32+14) /* "" Indirect Branch Restricted Speculation */
 #define X86_FEATURE_AMD_STIBP	(13*32+15) /* "" Single Thread Indirect Branch Predictors */
+#define X86_FEATURE_AMD_SSBD	(13*32+24) /* "" Speculative Store Bypass Disable */
 #define X86_FEATURE_VIRT_SSBD	(13*32+25) /* Virtualized Speculative Store Bypass Disable */
 #define X86_FEATURE_AMD_SSB_NO	(13*32+26) /* "" Speculative Store Bypass is fixed in hardware. */
 

arch/x86/kernel/cpu/bugs.c

@@ -531,18 +531,20 @@ static enum ssb_mitigation __init __ssb_select_mitigation(void)
 	if (mode == SPEC_STORE_BYPASS_DISABLE) {
 		setup_force_cpu_cap(X86_FEATURE_SPEC_STORE_BYPASS_DISABLE);
 		/*
-		 * Intel uses the SPEC CTRL MSR Bit(2) for this, while AMD uses
-		 * a completely different MSR and bit dependent on family.
+		 * Intel uses the SPEC CTRL MSR Bit(2) for this, while AMD may
+		 * use a completely different MSR and bit dependent on family.
 		 */
 		switch (boot_cpu_data.x86_vendor) {
 		case X86_VENDOR_INTEL:
+		case X86_VENDOR_AMD:
+			if (!static_cpu_has(X86_FEATURE_MSR_SPEC_CTRL)) {
+				x86_amd_ssb_disable();
+				break;
+			}
 			x86_spec_ctrl_base |= SPEC_CTRL_SSBD;
 			x86_spec_ctrl_mask |= SPEC_CTRL_SSBD;
 			wrmsrl(MSR_IA32_SPEC_CTRL, x86_spec_ctrl_base);
 			break;
-		case X86_VENDOR_AMD:
-			x86_amd_ssb_disable();
-			break;
 		}
 	}
 

arch/x86/kernel/cpu/common.c

@@ -752,6 +752,12 @@ static void init_speculation_control(struct cpuinfo_x86 *c)
 		set_cpu_cap(c, X86_FEATURE_STIBP);
 		set_cpu_cap(c, X86_FEATURE_MSR_SPEC_CTRL);
 	}
+
+	if (cpu_has(c, X86_FEATURE_AMD_SSBD)) {
+		set_cpu_cap(c, X86_FEATURE_SSBD);
+		set_cpu_cap(c, X86_FEATURE_MSR_SPEC_CTRL);
+		clear_cpu_cap(c, X86_FEATURE_VIRT_SSBD);
+	}
 }
 
 void get_cpu_cap(struct cpuinfo_x86 *c)

arch/x86/kvm/cpuid.c

@@ -355,7 +355,8 @@ static inline int __do_cpuid_ent(struct kvm_cpuid_entry2 *entry, u32 function,
 
 	/* cpuid 0x80000008.ebx */
 	const u32 kvm_cpuid_8000_0008_ebx_x86_features =
-		F(AMD_IBPB) | F(AMD_IBRS) | F(VIRT_SSBD) | F(AMD_SSB_NO);
+		F(AMD_IBPB) | F(AMD_IBRS) | F(AMD_SSBD) | F(VIRT_SSBD) |
+		F(AMD_SSB_NO);
 
 	/* cpuid 0xC0000001.edx */
 	const u32 kvm_cpuid_C000_0001_edx_x86_features =
@@ -633,7 +634,12 @@ static inline int __do_cpuid_ent(struct kvm_cpuid_entry2 *entry, u32 function,
 			entry->ebx |= F(VIRT_SSBD);
 		entry->ebx &= kvm_cpuid_8000_0008_ebx_x86_features;
 		cpuid_mask(&entry->ebx, CPUID_8000_0008_EBX);
-		if (boot_cpu_has(X86_FEATURE_LS_CFG_SSBD))
+		/*
+		 * The preference is to use SPEC CTRL MSR instead of the
+		 * VIRT_SPEC MSR.
+		 */
+		if (boot_cpu_has(X86_FEATURE_LS_CFG_SSBD) &&
+		    !boot_cpu_has(X86_FEATURE_AMD_SSBD))
 			entry->ebx |= F(VIRT_SSBD);
 		break;
 	}

arch/x86/kvm/cpuid.h

@@ -176,7 +176,7 @@ static inline bool guest_cpuid_has_spec_ctrl(struct kvm_vcpu *vcpu)
 	struct kvm_cpuid_entry2 *best;
 
 	best = kvm_find_cpuid_entry(vcpu, 0x80000008, 0);
-	if (best && (best->ebx & bit(X86_FEATURE_AMD_IBRS)))
+	if (best && (best->ebx & (bit(X86_FEATURE_AMD_IBRS | bit(X86_FEATURE_AMD_SSBD)))))
 		return true;
 	best = kvm_find_cpuid_entry(vcpu, 7, 0);
 	return best && (best->edx & (bit(X86_FEATURE_SPEC_CTRL) | bit(X86_FEATURE_SPEC_CTRL_SSBD)));

arch/x86/kvm/svm.c

@@ -3704,7 +3704,7 @@ static int svm_set_msr(struct kvm_vcpu *vcpu, struct msr_data *msr)
 			return 1;
 
 		/* The STIBP bit doesn't fault even if it's not advertised */
-		if (data & ~(SPEC_CTRL_IBRS | SPEC_CTRL_STIBP))
+		if (data & ~(SPEC_CTRL_IBRS | SPEC_CTRL_STIBP | SPEC_CTRL_SSBD))
 			return 1;
 
 		svm->spec_ctrl = data;