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[Xen-devel] [PATCH] 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 the
initial initrd mapping, even if it 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@xxxxxxxxxx>
CC: Jan Beulich <JBeulich@xxxxxxxx>
CC: Tim Deegan <tim@xxxxxxx>
CC: George Dunlap <george.dunlap@xxxxxxxxxxxxx>
CC: Wei Liu <wei.liu2@xxxxxxxxxx>
 xen/include/asm-x86/guest_pt.h | 12 +++++++-----
 1 file changed, 7 insertions(+), 5 deletions(-)

diff --git a/xen/include/asm-x86/guest_pt.h b/xen/include/asm-x86/guest_pt.h
index 72126d5..08031c8 100644
--- 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)

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