x86: command line option to avoid use of secondary hyper-threads

Shared resources (L1 cache and TLB in particular) present a risk of
information leak via side channels. Provide a means to avoid use of
hyperthreads in such cases.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Roger Pau Monné <roger.pau@citrix.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>
master commit: d8f974f1a646c0200b97ebcabb808324b288fadb
master date: 2018-07-19 13:43:33 +0100

diff --git a/docs/misc/xen-command-line.markdown b/docs/misc/xen-command-line.markdown
index cb81a9cbc4678599b427bb6f57bd3351ba2718ea..dba37e2e5331c56abd554b0bee428e187d315ae7 100644 (file)
--- a/docs/misc/xen-command-line.markdown
+++ b/docs/misc/xen-command-line.markdown
@@ -1717,6 +1717,13 @@ Use `smap=hvm` to allow SMAP use by HVM guests only.
 Flag to enable Supervisor Mode Execution Protection
 Use `smep=hvm` to allow SMEP use by HVM guests only.
 
+### smt (x86)
+> `= <boolean>`
+
+Default: `true`
+
+Control bring up of multiple hyper-threads per CPU core.
+
 ### snb\_igd\_quirk
 > `= <boolean> | cap | <integer>`
 

diff --git a/xen/arch/x86/setup.c b/xen/arch/x86/setup.c
index 5c384d8a358887851606277a759761d64326ab4b..7cce57a2741f2360e708939ba6e7489dfa2377bb 100644 (file)
--- a/xen/arch/x86/setup.c
+++ b/xen/arch/x86/setup.c
@@ -62,6 +62,9 @@ boolean_param("nosmp", opt_nosmp);
 static unsigned int __initdata max_cpus;
 integer_param("maxcpus", max_cpus);
 
+int8_t __read_mostly opt_smt = -1;
+boolean_param("smt", opt_smt);
+
 /* opt_invpcid: If false, don't use INVPCID instruction even if available. */
 static bool __initdata opt_invpcid = true;
 boolean_param("invpcid", opt_invpcid);
@@ -1617,7 +1620,10 @@ void __init noreturn __start_xen(unsigned long mbi_p)
                 int ret = cpu_up(i);
                 if ( ret != 0 )
                     printk("Failed to bring up CPU %u (error %d)\n", i, ret);
-                else if ( num_online_cpus() > max_cpus )
+                else if ( num_online_cpus() > max_cpus ||
+                          (!opt_smt &&
+                           cpu_data[i].compute_unit_id == INVALID_CUID &&
+                           cpumask_weight(per_cpu(cpu_sibling_mask, i)) > 1) )
                 {
                     ret = cpu_down(i);
                     if ( !ret )

diff --git a/xen/arch/x86/sysctl.c b/xen/arch/x86/sysctl.c
index 6ba823ca692c95e66aaf69a0aa7364a857905ee3..e726eee9741b687faef21c2e44aee9fcbc409cca 100644 (file)
--- a/xen/arch/x86/sysctl.c
+++ b/xen/arch/x86/sysctl.c
@@ -23,6 +23,7 @@
 #include <asm/hvm/hvm.h>
 #include <asm/hvm/support.h>
 #include <asm/processor.h>
+#include <asm/setup.h>
 #include <asm/smp.h>
 #include <asm/numa.h>
 #include <xen/nodemask.h>
@@ -48,14 +49,27 @@ static void l3_cache_get(void *arg)
 
 long cpu_up_helper(void *data)
 {
-    int cpu = (unsigned long)data;
+    unsigned int cpu = (unsigned long)data;
     int ret = cpu_up(cpu);
+
     if ( ret == -EBUSY )
     {
         /* On EBUSY, flush RCU work and have one more go. */
         rcu_barrier();
         ret = cpu_up(cpu);
     }
+
+    if ( !ret && !opt_smt &&
+         cpu_data[cpu].compute_unit_id == INVALID_CUID &&
+         cpumask_weight(per_cpu(cpu_sibling_mask, cpu)) > 1 )
+    {
+        ret = cpu_down_helper(data);
+        if ( ret )
+            printk("Could not re-offline CPU%u (%d)\n", cpu, ret);
+        else
+            ret = -EPERM;
+    }
+
     return ret;
 }
 

diff --git a/xen/include/asm-x86/setup.h b/xen/include/asm-x86/setup.h
index b68ec9de4d76d28df930ce6c24e1a02b4e8a17d0..ecfd0c2e7baf454f3689f0b89b4f56b0cd054204 100644 (file)
--- a/xen/include/asm-x86/setup.h
+++ b/xen/include/asm-x86/setup.h
@@ -66,6 +66,8 @@ extern uint8_t kbd_shift_flags;
 extern unsigned long highmem_start;
 #endif
 
+extern int8_t opt_smt;
+
 #ifdef CONFIG_SHADOW_PAGING
 extern bool opt_dom0_shadow;
 #else