[Xen-changelog] [xen staging-4.8] x86: command line option to avoid use of secondary hyper-threads

[patchbot@xxxxxxx]

commit f049cd67a99bcf773aa4fceeedd5d1de17b2a8eb
Author:     Jan Beulich <jbeulich@xxxxxxxx>
AuthorDate: Mon Jul 30 12:08:14 2018 +0200
Commit:     Jan Beulich <jbeulich@xxxxxxxx>
CommitDate: Mon Jul 30 12:08:14 2018 +0200

    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@xxxxxxxx>
    Reviewed-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
    Reviewed-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
    master commit: d8f974f1a646c0200b97ebcabb808324b288fadb
    master date: 2018-07-19 13:43:33 +0100
---
 docs/misc/xen-command-line.markdown |  7 +++++++
 xen/arch/x86/setup.c                |  8 +++++++-
 xen/arch/x86/sysctl.c               | 16 +++++++++++++++-
 xen/include/asm-x86/setup.h         |  2 ++
 4 files changed, 31 insertions(+), 2 deletions(-)

diff --git a/docs/misc/xen-command-line.markdown 
b/docs/misc/xen-command-line.markdown
index 7a95744f62..67a2fa4e2d 100644
--- a/docs/misc/xen-command-line.markdown
+++ b/docs/misc/xen-command-line.markdown
@@ -1534,6 +1534,13 @@ 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 fd80b6dfef..b857a8646d 100644
--- 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);
@@ -1554,7 +1557,10 @@ 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 14e7dc7680..b7a7e85ae7 100644
--- a/xen/arch/x86/sysctl.c
+++ b/xen/arch/x86/sysctl.c
@@ -24,6 +24,7 @@
 #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>
@@ -49,14 +50,27 @@ 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 c65b79c6f9..047637d98c 100644
--- a/xen/include/asm-x86/setup.h
+++ b/xen/include/asm-x86/setup.h
@@ -57,4 +57,6 @@ extern uint8_t kbd_shift_flags;
 extern unsigned long highmem_start;
 #endif
 
+extern int8_t opt_smt;
+
 #endif
--
generated by git-patchbot for /home/xen/git/xen.git#staging-4.8

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