x86/pagewalk: Remove opt_allow_superpage check from guest_can_use_l2_superpages()

The purpose of guest_walk_tables() is to match the behaviour of real hardware.

A PV guest can have 2M superpages in its pagetables, via the M2P (and for dom0
via the initial P2M), even if the guest isn't permitted to create arbitrary 2M
superpage mappings.

guest_can_use_l2_superpages() checking opt_allow_superpage is a piece of PV
guest policy enforcement, rather than its intended purpose of meaning "would
hardware tolerate finding an L2 superpage with these control settings?"

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Tim Deegan <tim@xen.org>
Reviewed-by: Wei Liu <wei.liu2@citrix.com>

diff --git a/xen/include/asm-x86/guest_pt.h b/xen/include/asm-x86/guest_pt.h
index 72126d58d5a843ecf9db5555e6c158dd5245fbfa..08031c803e1432af0a6c61392872af66f4c57ddd 100644 (file)
--- a/xen/include/asm-x86/guest_pt.h
+++ b/xen/include/asm-x86/guest_pt.h
@@ -205,15 +205,17 @@ static inline guest_l4e_t guest_l4e_from_gfn(gfn_t gfn, u32 flags)
 static inline bool guest_can_use_l2_superpages(const struct vcpu *v)
 {
     /*
+     * PV guests use Xen's paging settings.  Being 4-level, 2M
+     * superpages are unconditionally supported.
+     *
      * The L2 _PAGE_PSE bit must be honoured in HVM guests, whenever
      * CR4.PSE is set or the guest is in PAE or long mode.
      * It's also used in the dummy PT for vcpus with CR0.PG cleared.
      */
-    return (is_pv_vcpu(v)
-            ? opt_allow_superpage
-            : (GUEST_PAGING_LEVELS != 2
-               || !hvm_paging_enabled(v)
-               || (v->arch.hvm_vcpu.guest_cr[4] & X86_CR4_PSE)));
+    return (is_pv_vcpu(v) ||
+            GUEST_PAGING_LEVELS != 2 ||
+            !hvm_paging_enabled(v) ||
+            (v->arch.hvm_vcpu.guest_cr[4] & X86_CR4_PSE));
 }
 
 static inline bool guest_can_use_l3_superpages(const struct domain *d)