x86/HVM: refine SMEP/SMAP tests in HVM_CR4_GUEST_RESERVED_BITS()

Andrew validly points out that the use of the macro on the restore path
can't rely on the CPUID bits for the guest already being in place (as
their setting by the tool stack in turn requires the other restore
operations already having taken place). And even worse, using
hvm_cpuid() is invalid here because that function assumes to be used in
the context of the vCPU in question.

Reverting to the behavior prior to the change from checking
cpu_has_sm?p to hvm_vcpu_has_sm?p() would break the other (non-restore)
use of the macro. So let's revert to the prior behavior only for the
restore path, by adding a respective second parameter to the macro.

Obviously the two cpu_has_* uses in the macro should really also be
converted to hvm_cpuid() based checks at least for the non-restore
path.

Reported-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>

--- a/xen/arch/x86/hvm/hvm.c
+++ b/xen/arch/x86/hvm/hvm.c
@@ -1754,7 +1754,7 @@ static int hvm_load_cpu_ctxt(struct doma
         return -EINVAL;
     }
 
-    if ( ctxt.cr4 & HVM_CR4_GUEST_RESERVED_BITS(v) )
+    if ( ctxt.cr4 & HVM_CR4_GUEST_RESERVED_BITS(v, 1) )
     {
         printk(XENLOG_G_ERR "HVM%d restore: bad CR4 %#" PRIx64 "\n",
                d->domain_id, ctxt.cr4);
@@ -3186,7 +3186,7 @@ int hvm_set_cr4(unsigned long value)
     struct vcpu *v = current;
     unsigned long old_cr;
 
-    if ( value & HVM_CR4_GUEST_RESERVED_BITS(v) )
+    if ( value & HVM_CR4_GUEST_RESERVED_BITS(v, 0) )
     {
         HVM_DBG_LOG(DBG_LEVEL_1,
                     "Guest attempts to set reserved bit in CR4: %lx",
--- a/xen/include/asm-x86/hvm/hvm.h
+++ b/xen/include/asm-x86/hvm/hvm.h
@@ -406,19 +406,27 @@ static inline bool_t hvm_vcpu_has_smep(v
     (X86_CR4_VMXE | X86_CR4_PAE | X86_CR4_MCE))
 
 /* These bits in CR4 cannot be set by the guest. */
-#define HVM_CR4_GUEST_RESERVED_BITS(_v)                 \
+#define HVM_CR4_GUEST_RESERVED_BITS(v, restore) ({      \
+    const struct vcpu *_v = (v);                        \
+    bool_t _restore = !!(restore);                      \
+    ASSERT((_restore) || _v == current);                \
     (~((unsigned long)                                  \
        (X86_CR4_VME | X86_CR4_PVI | X86_CR4_TSD |       \
         X86_CR4_DE  | X86_CR4_PSE | X86_CR4_PAE |       \
         X86_CR4_MCE | X86_CR4_PGE | X86_CR4_PCE |       \
         X86_CR4_OSFXSR | X86_CR4_OSXMMEXCPT |           \
-        (hvm_vcpu_has_smep() ? X86_CR4_SMEP : 0) |      \
-        (hvm_vcpu_has_smap() ? X86_CR4_SMAP : 0) |      \
+        (((_restore) ? cpu_has_smep :                   \
+                       hvm_vcpu_has_smep()) ?           \
+         X86_CR4_SMEP : 0) |                            \
+        (((_restore) ? cpu_has_smap :                   \
+                       hvm_vcpu_has_smap()) ?           \
+         X86_CR4_SMAP : 0) |                            \
         (cpu_has_fsgsbase ? X86_CR4_FSGSBASE : 0) |     \
-        ((nestedhvm_enabled((_v)->domain) && cpu_has_vmx)\
-                      ? X86_CR4_VMXE : 0)  |             \
-        (cpu_has_pcid ? X86_CR4_PCIDE : 0) |             \
-        (cpu_has_xsave ? X86_CR4_OSXSAVE : 0))))
+        ((nestedhvm_enabled(_v->domain) && cpu_has_vmx) \
+                      ? X86_CR4_VMXE : 0)  |            \
+        (cpu_has_pcid ? X86_CR4_PCIDE : 0) |            \
+        (cpu_has_xsave ? X86_CR4_OSXSAVE : 0))));       \
+})
 
 /* These exceptions must always be intercepted. */
 #define HVM_TRAP_MASK ((1U << TRAP_machine_check) | (1U << TRAP_invalid_op))