x86/amd: Use newer SSBD mechanisms if they exist

The opencoded legacy Memory Disambiguation logic in init_amd() neglected
Fam19h for the Zen3 microarchitecture.  Further more, all Zen2 based system
have the architectural MSR_SPEC_CTRL and the SSBD bit within it, so shouldn't
be using MSR_AMD64_LS_CFG.

Implement the algorithm given in AMD's SSBD whitepaper, and leave a
printk_once() behind in the case that no controls can be found.

This now means that a user explicitly choosing `spec-ctrl=ssbd` will properly
turn off Memory Disambiguation on Fam19h/Zen3 systems.

This still remains a single system-wide setting (for now), and is not context
switched between vCPUs.  As such, it doesn't interact with Intel's use of
MSR_SPEC_CTRL and default_xen_spec_ctrl (yet).

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
master commit: 2a4e6c4e4b
master date: 2021-09-08 14:16:19 +0100

xen/arch/x86/cpu/amd.c

@@ -642,6 +642,56 @@ void early_init_amd(struct cpuinfo_x86 *c)
 	ctxt_switch_levelling(NULL);
 }
 
+/*
+ * Refer to the AMD Speculative Store Bypass whitepaper:
+ * https://developer.amd.com/wp-content/resources/124441_AMD64_SpeculativeStoreBypassDisable_Whitepaper_final.pdf
+ */
+void amd_init_ssbd(const struct cpuinfo_x86 *c)
+{
+	int bit = -1;
+
+	if (cpu_has_ssb_no)
+		return;
+
+	if (cpu_has_amd_ssbd) {
+		wrmsrl(MSR_SPEC_CTRL, opt_ssbd ? SPEC_CTRL_SSBD : 0);
+		return;
+	}
+
+	if (cpu_has_virt_ssbd) {
+		wrmsrl(MSR_VIRT_SPEC_CTRL, opt_ssbd ? SPEC_CTRL_SSBD : 0);
+		return;
+	}
+
+	switch (c->x86) {
+	case 0x15: bit = 54; break;
+	case 0x16: bit = 33; break;
+	case 0x17:
+	case 0x18: bit = 10; break;
+	}
+
+	if (bit >= 0) {
+		uint64_t val, mask = 1ull << bit;
+
+		if (rdmsr_safe(MSR_AMD64_LS_CFG, val) ||
+		    ({
+			    val &= ~mask;
+			    if (opt_ssbd)
+				    val |= mask;
+			    false;
+		    }) ||
+		    wrmsr_safe(MSR_AMD64_LS_CFG, val) ||
+		    ({
+			    rdmsrl(MSR_AMD64_LS_CFG, val);
+			    (val & mask) != (opt_ssbd * mask);
+		    }))
+			bit = -1;
+	}
+
+	if (bit < 0)
+		printk_once(XENLOG_ERR "No SSBD controls available\n");
+}
+
 static void init_amd(struct cpuinfo_x86 *c)
 {
 	u32 l, h;
@@ -718,24 +768,7 @@ static void init_amd(struct cpuinfo_x86 *c)
 				  c->x86_capability);
 	}
 
-	/*
-	 * If the user has explicitly chosen to disable Memory Disambiguation
-	 * to mitigiate Speculative Store Bypass, poke the appropriate MSR.
-	 */
-	if (opt_ssbd) {
-		int bit = -1;
-
-		switch (c->x86) {
-		case 0x15: bit = 54; break;
-		case 0x16: bit = 33; break;
-		case 0x17: bit = 10; break;
-		}
-
-		if (bit >= 0 && !rdmsr_safe(MSR_AMD64_LS_CFG, value)) {
-			value |= 1ull << bit;
-			wrmsr_safe(MSR_AMD64_LS_CFG, value);
-		}
-	}
+	amd_init_ssbd(c);
 
 	/* MFENCE stops RDTSC speculation */
 	if (!cpu_has_lfence_dispatch)

xen/arch/x86/cpu/cpu.h

@@ -20,3 +20,4 @@ extern bool detect_extended_topology(struct cpuinfo_x86 *c);
 
 void early_init_amd(struct cpuinfo_x86 *c);
 void amd_log_freq(const struct cpuinfo_x86 *c);
+void amd_init_ssbd(const struct cpuinfo_x86 *c);

xen/arch/x86/cpu/hygon.c

@@ -59,14 +59,7 @@ static void init_hygon(struct cpuinfo_x86 *c)
 		__set_bit(X86_FEATURE_LFENCE_DISPATCH,
 			  c->x86_capability);
 
-	/*
-	 * If the user has explicitly chosen to disable Memory Disambiguation
-	 * to mitigiate Speculative Store Bypass, poke the appropriate MSR.
-	 */
-	if (opt_ssbd && !rdmsr_safe(MSR_AMD64_LS_CFG, value)) {
-		value |= 1ull << 10;
-		wrmsr_safe(MSR_AMD64_LS_CFG, value);
-	}
+	amd_init_ssbd(c);
 
 	/* MFENCE stops RDTSC speculation */
 	if (!cpu_has_lfence_dispatch)

xen/arch/x86/spec_ctrl.c

@@ -326,20 +326,23 @@ static void __init print_details(enum ind_thunk thunk, uint64_t caps)
            (caps & ARCH_CAPS_IBRS_ALL)                       ? " IBRS_ALL"       : "",
            (caps & ARCH_CAPS_RSBA)                           ? " RSBA"           : "",
            (caps & ARCH_CAPS_SKIP_L1DFL)                     ? " SKIP_L1DFL"     : "",
+           (e8b  & cpufeat_mask(X86_FEATURE_SSB_NO)) ||
            (caps & ARCH_CAPS_SSB_NO)                         ? " SSB_NO"         : "",
            (caps & ARCH_CAPS_MDS_NO)                         ? " MDS_NO"         : "",
            (caps & ARCH_CAPS_TAA_NO)                         ? " TAA_NO"         : "");
 
     /* Hardware features which need driving to mitigate issues. */
-    printk("  Hardware features:%s%s%s%s%s%s%s%s\n",
+    printk("  Hardware features:%s%s%s%s%s%s%s%s%s\n",
            (e8b  & cpufeat_mask(X86_FEATURE_IBPB)) ||
            (_7d0 & cpufeat_mask(X86_FEATURE_IBRSB))          ? " IBPB"           : "",
            (_7d0 & cpufeat_mask(X86_FEATURE_IBRSB))          ? " IBRS"           : "",
            (_7d0 & cpufeat_mask(X86_FEATURE_STIBP))          ? " STIBP"          : "",
+           (e8b  & cpufeat_mask(X86_FEATURE_AMD_SSBD)) ||
            (_7d0 & cpufeat_mask(X86_FEATURE_SSBD))           ? " SSBD"           : "",
            (_7d0 & cpufeat_mask(X86_FEATURE_L1D_FLUSH))      ? " L1D_FLUSH"      : "",
            (_7d0 & cpufeat_mask(X86_FEATURE_MD_CLEAR))       ? " MD_CLEAR"       : "",
            (_7d0 & cpufeat_mask(X86_FEATURE_SRBDS_CTRL))     ? " SRBDS_CTRL"     : "",
+           (e8b  & cpufeat_mask(X86_FEATURE_VIRT_SSBD))      ? " VIRT_SSBD"      : "",
            (caps & ARCH_CAPS_TSX_CTRL)                       ? " TSX_CTRL"       : "");
 
     /* Compiled-in support which pertains to mitigations. */