[PATCH 7/9] x86/hvm: Disable MPX by default

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

Memory Protection eXtension support has been dropped from GCC and Linux, and
will be dropped from future Intel CPUs.

With all other default/max pieces in place, move MPX from default to max.
This means that VMs won't be offered it by default, but can explicitly opt
into using it via cpuid="host,mpx=1" in their vm.cfg file.

The difference as visible to the guest is:

  diff --git a/default b/mpx
  index 0e91765d6b..c8c33cd584 100644
  --- a/default
  +++ b/mpx
  @@ -13,15 +13,17 @@ Native cpuid:
     00000004:00000004 -> 00000000:00000000:00000000:00000000
     00000005:ffffffff -> 00000000:00000000:00000000:00000000
     00000006:ffffffff -> 00000000:00000000:00000000:00000000
  -  00000007:00000000 -> 00000000:009c2fbb:00000000:9c000400
  +  00000007:00000000 -> 00000000:009c6fbb:00000000:9c000400
     00000008:ffffffff -> 00000000:00000000:00000000:00000000
     00000009:ffffffff -> 00000000:00000000:00000000:00000000
     0000000a:ffffffff -> 00000000:00000000:00000000:00000000
     0000000b:ffffffff -> 00000000:00000000:00000000:00000000
     0000000c:ffffffff -> 00000000:00000000:00000000:00000000
  -  0000000d:00000000 -> 00000007:00000240:00000340:00000000
  +  0000000d:00000000 -> 0000001f:00000240:00000440:00000000
     0000000d:00000001 -> 0000000f:00000240:00000000:00000000
     0000000d:00000002 -> 00000100:00000240:00000000:00000000
  +  0000000d:00000003 -> 00000040:000003c0:00000000:00000000
  +  0000000d:00000004 -> 00000040:00000400:00000000:00000000
     40000000:ffffffff -> 40000005:566e6558:65584d4d:4d4d566e
     40000001:ffffffff -> 0004000e:00000000:00000000:00000000
     40000002:ffffffff -> 00000001:40000000:00000000:00000000

Adjust the legacy restore path in libxc to cope safely with pre-4.14 VMs.

Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
---
CC: Jan Beulich <JBeulich@xxxxxxxx>
CC: Wei Liu <wl@xxxxxxx>
CC: Roger Pau Monné <roger.pau@xxxxxxxxxx>
CC: Ian Jackson <Ian.Jackson@xxxxxxxxxx>
CC: Wei Liu <wl@xxxxxxx>
CC: Paul Durrant <paul@xxxxxxx>

Dropped Jan's R-by from previous posting, as the patch has gained new
toolstack logic to avoid breaking migrate.
---
 tools/libxc/xc_cpuid_x86.c                  | 48 ++++++++++++++++++-----------
 xen/include/public/arch-x86/cpufeatureset.h |  2 +-
 2 files changed, 31 insertions(+), 19 deletions(-)

diff --git a/tools/libxc/xc_cpuid_x86.c b/tools/libxc/xc_cpuid_x86.c
index e017abffce..5649913e69 100644
--- a/tools/libxc/xc_cpuid_x86.c
+++ b/tools/libxc/xc_cpuid_x86.c
@@ -436,6 +436,8 @@ int xc_cpuid_apply_policy(xc_interface *xch, uint32_t 
domid, bool restore,
     xen_cpuid_leaf_t *leaves = NULL;
     struct cpuid_policy *p = NULL;
     uint32_t err_leaf = -1, err_subleaf = -1, err_msr = -1;
+    uint32_t host_featureset[FEATURESET_NR_ENTRIES] = {};
+    uint32_t len = ARRAY_SIZE(host_featureset);
 
     if ( xc_domain_getinfo(xch, domid, 1, &di) != 1 ||
          di.domid != domid )
@@ -458,6 +460,22 @@ int xc_cpuid_apply_policy(xc_interface *xch, uint32_t 
domid, bool restore,
          (p = calloc(1, sizeof(*p))) == NULL )
         goto out;
 
+    /* Get the host policy. */
+    rc = xc_get_cpu_featureset(xch, XEN_SYSCTL_cpu_featureset_host,
+                               &len, host_featureset);
+    if ( rc )
+    {
+        /* Tolerate "buffer too small", as we've got the bits we need. */
+        if ( errno == ENOBUFS )
+            rc = 0;
+        else
+        {
+            PERROR("Failed to obtain host featureset");
+            rc = -errno;
+            goto out;
+        }
+    }
+
     /* Get the domain's default policy. */
     nr_msrs = 0;
     rc = xc_get_system_cpu_policy(xch, di.hvm ? 
XEN_SYSCTL_cpu_policy_hvm_default
@@ -479,6 +497,18 @@ int xc_cpuid_apply_policy(xc_interface *xch, uint32_t 
domid, bool restore,
         goto out;
     }
 
+    /*
+     * Account for feature which have been disabled by default since Xen 4.13,
+     * so migrated-in VM's don't risk seeing features disappearing.
+     */
+    if ( restore )
+    {
+        if ( di.hvm )
+        {
+            p->feat.mpx = test_bit(X86_FEATURE_MPX, host_featureset);
+        }
+    }
+
     if ( featureset )
     {
         uint32_t disabled_features[FEATURESET_NR_ENTRIES],
@@ -530,24 +560,6 @@ int xc_cpuid_apply_policy(xc_interface *xch, uint32_t 
domid, bool restore,
 
     if ( !di.hvm )
     {
-        uint32_t host_featureset[FEATURESET_NR_ENTRIES] = {};
-        uint32_t len = ARRAY_SIZE(host_featureset);
-
-        rc = xc_get_cpu_featureset(xch, XEN_SYSCTL_cpu_featureset_host,
-                                   &len, host_featureset);
-        if ( rc )
-        {
-            /* Tolerate "buffer too small", as we've got the bits we need. */
-            if ( errno == ENOBUFS )
-                rc = 0;
-            else
-            {
-                PERROR("Failed to obtain host featureset");
-                rc = -errno;
-                goto out;
-            }
-        }
-
         /*
          * On hardware without CPUID Faulting, PV guests see real topology.
          * As a consequence, they also need to see the host htt/cmp fields.
diff --git a/xen/include/public/arch-x86/cpufeatureset.h 
b/xen/include/public/arch-x86/cpufeatureset.h
index 5ca35d9d97..af1b8a96a6 100644
--- a/xen/include/public/arch-x86/cpufeatureset.h
+++ b/xen/include/public/arch-x86/cpufeatureset.h
@@ -207,7 +207,7 @@ XEN_CPUFEATURE(INVPCID,       5*32+10) /*H  Invalidate 
Process Context ID */
 XEN_CPUFEATURE(RTM,           5*32+11) /*A  Restricted Transactional Memory */
 XEN_CPUFEATURE(PQM,           5*32+12) /*   Platform QoS Monitoring */
 XEN_CPUFEATURE(NO_FPU_SEL,    5*32+13) /*!  FPU CS/DS stored as zero */
-XEN_CPUFEATURE(MPX,           5*32+14) /*S  Memory Protection Extensions */
+XEN_CPUFEATURE(MPX,           5*32+14) /*s  Memory Protection Extensions */
 XEN_CPUFEATURE(PQE,           5*32+15) /*   Platform QoS Enforcement */
 XEN_CPUFEATURE(AVX512F,       5*32+16) /*A  AVX-512 Foundation Instructions */
 XEN_CPUFEATURE(AVX512DQ,      5*32+17) /*A  AVX-512 Doubleword & Quadword 
Instrs */
-- 
2.11.0