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[Xen-devel] [PATCH] tools/libxc: Fix HVM_PARAM_PAE_ENABLED handling in xc_cpuid_apply_policy()

Despite as suggested in c/s 685e922d6f3, not all HVM_PARAMs are handled
in the same way.  HVM_PARAM_PAE_ENABLED is a toolstack-only value, and
the xc_cpuid_apply_policy() used to be the only consumer.

Reinstate the old behaviour (mad as it is) to avoid regressions.

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>

Spotted while trying to come up with a less mad behaviour, and wondering
where the consumer had disappaered to.

This needs backporting to 4.13.

I was tempted to not fix it for the general security betterness of the
world (disabling PAE disables NX and 64bit), and while I could probably
get away with it, it would a little rude to do so.
 tools/libxc/xc_cpuid_x86.c | 12 ++++++++++++
 1 file changed, 12 insertions(+)

diff --git a/tools/libxc/xc_cpuid_x86.c b/tools/libxc/xc_cpuid_x86.c
index 519d6d8bd0..2540aa1e1c 100644
--- a/tools/libxc/xc_cpuid_x86.c
+++ b/tools/libxc/xc_cpuid_x86.c
@@ -579,6 +579,8 @@ int xc_cpuid_apply_policy(xc_interface *xch, uint32_t domid,
+        uint64_t val;
          * Topology for HVM guests is entirely controlled by Xen.  For now, we
          * hardcode APIC_ID = vcpu_id * 2 to give the illusion of no SMT.
@@ -633,6 +635,16 @@ int xc_cpuid_apply_policy(xc_interface *xch, uint32_t 
+         * HVM_PARAM_PAE_ENABLED is a parameter to this function, stashed in
+         * Xen.  Nothing else has ever taken notice of the value.
+         */
+        rc = xc_hvm_param_get(xch, domid, HVM_PARAM_PAE_ENABLED, &val);
+        if ( rc )
+            goto out;
+        p->basic.pae = val;
+        /*
          * These settings are necessary to cause earlier HVM_PARAM_NESTEDHVM /
          * XEN_DOMCTL_disable_migrate settings to be reflected correctly in
          * CPUID.  Xen will discard these bits if configuration hasn't been

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