x86-64: refine the XSA-9 fix

Our product management wasn't happy with the "solution" for XSA-9, and
demanded that customer systems must continue to boot. Rather than
having our and perhaps other distros carry non-trivial patches, allow
for more fine grained control (panic on boot, deny guest creation, or
merely warn) by means of a single line change.

Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>

--- a/xen/arch/x86/cpu/amd.c
+++ b/xen/arch/x86/cpu/amd.c
@@ -32,8 +32,11 @@
 static char opt_famrev[14];
 string_param("cpuid_mask_cpu", opt_famrev);
 
-static bool_t opt_allow_unsafe;
+#ifdef __x86_64__
+/* 1 = allow, 0 = don't allow guest creation, -1 = don't allow boot */
+s8 __read_mostly opt_allow_unsafe = -1;
 boolean_param("allow_unsafe", opt_allow_unsafe);
+#endif
 
 static inline void wrmsr_amd(unsigned int index, unsigned int lo, 
 		unsigned int hi)
@@ -496,10 +499,19 @@ static void __devinit init_amd(struct cp
 		clear_bit(X86_FEATURE_MWAIT, c->x86_capability);
 
 #ifdef __x86_64__
-	if (cpu_has_amd_erratum(c, AMD_ERRATUM_121) && !opt_allow_unsafe)
+	if (!cpu_has_amd_erratum(c, AMD_ERRATUM_121))
+		opt_allow_unsafe = 1;
+	else if (opt_allow_unsafe < 0)
 		panic("Xen will not boot on this CPU for security reasons.\n"
 		      "Pass \"allow_unsafe\" if you're trusting all your"
 		      " (PV) guest kernels.\n");
+	else if (!opt_allow_unsafe && c == &boot_cpu_data)
+		printk(KERN_WARNING
+		       "*** Xen will not allow creation of DomU-s on"
+		       " this CPU for security reasons. ***\n"
+		       KERN_WARNING
+		       "*** Pass \"allow_unsafe\" if you're trusting"
+		       " all your (PV) guest kernels. ***\n");
 
 	/* AMD CPUs do not support SYSENTER outside of legacy mode. */
 	clear_bit(X86_FEATURE_SEP, c->x86_capability);
--- a/xen/arch/x86/domain.c
+++ b/xen/arch/x86/domain.c
@@ -55,6 +55,7 @@
 #include <asm/traps.h>
 #include <asm/nmi.h>
 #include <asm/mce.h>
+#include <asm/amd.h>
 #include <xen/numa.h>
 #include <xen/iommu.h>
 #ifdef CONFIG_COMPAT
@@ -531,6 +532,20 @@ int arch_domain_create(struct domain *d,
 
 #else /* __x86_64__ */
 
+    if ( d->domain_id && !is_idle_domain(d) &&
+         cpu_has_amd_erratum(&boot_cpu_data, AMD_ERRATUM_121) )
+    {
+        if ( !opt_allow_unsafe )
+        {
+            printk(XENLOG_G_ERR "Xen does not allow DomU creation on this CPU"
+                   " for security reasons.\n");
+            return -EPERM;
+        }
+        printk(XENLOG_G_WARNING
+               "Dom%d may compromise security on this CPU.\n",
+               d->domain_id);
+    }
+
     BUILD_BUG_ON(PDPT_L2_ENTRIES * sizeof(*d->arch.mm_perdomain_pt_pages)
                  != PAGE_SIZE);
     pg = alloc_domheap_page(NULL, MEMF_node(domain_to_node(d)));
--- a/xen/include/asm-x86/amd.h
+++ b/xen/include/asm-x86/amd.h
@@ -147,6 +147,8 @@ struct cpuinfo_x86;
 int cpu_has_amd_erratum(const struct cpuinfo_x86 *, int, ...);
 
 #ifdef __x86_64__
+extern s8 opt_allow_unsafe;
+
 void fam10h_check_enable_mmcfg(void);
 void check_enable_amd_mmconf_dmi(void);
 #endif