x86/spec-ctrl: Enable Zen2 chickenbit

... as instructed in the Branch Type Confusion whitepaper.

This is part of XSA-407.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>

xen/arch/x86/cpu/amd.c

@@ -731,6 +731,31 @@ void amd_init_ssbd(const struct cpuinfo_x86 *c)
 		printk_once(XENLOG_ERR "No SSBD controls available\n");
 }
 
+/*
+ * On Zen2 we offer this chicken (bit) on the altar of Speculation.
+ *
+ * Refer to the AMD Branch Type Confusion whitepaper:
+ * https://XXX
+ *
+ * Setting this unnamed bit supposedly causes prediction information on
+ * non-branch instructions to be ignored.  It is to be set unilaterally in
+ * newer microcode.
+ *
+ * This chickenbit is something unrelated on Zen1, and Zen1 vs Zen2 isn't a
+ * simple model number comparison, so use STIBP as a heuristic to separate the
+ * two uarches in Fam17h(AMD)/18h(Hygon).
+ */
+void amd_init_spectral_chicken(void)
+{
+	uint64_t val, chickenbit = 1 << 1;
+
+	if (cpu_has_hypervisor || !boot_cpu_has(X86_FEATURE_AMD_STIBP))
+		return;
+
+	if (rdmsr_safe(MSR_AMD64_DE_CFG2, val) == 0 && !(val & chickenbit))
+		wrmsr_safe(MSR_AMD64_DE_CFG2, val | chickenbit);
+}
+
 void __init detect_zen2_null_seg_behaviour(void)
 {
 	uint64_t base;
@@ -796,6 +821,9 @@ static void cf_check init_amd(struct cpuinfo_x86 *c)
 
 	amd_init_ssbd(c);
 
+	if (c->x86 == 0x17)
+		amd_init_spectral_chicken();
+
 	/* Probe for NSCB on Zen2 CPUs when not virtualised */
 	if (!cpu_has_hypervisor && !cpu_has_nscb && c == &boot_cpu_data &&
 	    c->x86 == 0x17)

xen/arch/x86/cpu/cpu.h

@@ -22,4 +22,5 @@ void cf_check early_init_amd(struct cpuinfo_x86 *c);
 void amd_log_freq(const struct cpuinfo_x86 *c);
 void amd_init_lfence(struct cpuinfo_x86 *c);
 void amd_init_ssbd(const struct cpuinfo_x86 *c);
+void amd_init_spectral_chicken(void);
 void detect_zen2_null_seg_behaviour(void);

xen/arch/x86/cpu/hygon.c

@@ -40,6 +40,12 @@ static void cf_check init_hygon(struct cpuinfo_x86 *c)
 	    c->x86 == 0x18)
 		detect_zen2_null_seg_behaviour();
 
+	/*
+	 * TODO: Check heuristic safety with Hygon first
+	if (c->x86 == 0x18)
+		amd_init_spectral_chicken();
+	 */
+
 	/*
 	 * Hygon CPUs before Zen2 don't clear segment bases/limits when
 	 * loading a NULL selector.

xen/arch/x86/include/asm/msr-index.h

@@ -377,6 +377,7 @@
 #define MSR_AMD64_DE_CFG		0xc0011029
 #define AMD64_DE_CFG_LFENCE_SERIALISE	(_AC(1, ULL) << 1)
 #define MSR_AMD64_EX_CFG		0xc001102c
+#define MSR_AMD64_DE_CFG2		0xc00110e3
 
 #define MSR_AMD64_DR0_ADDRESS_MASK	0xc0011027
 #define MSR_AMD64_DR1_ADDRESS_MASK	0xc0011019