x86/cpuid: leak OSXSAVE only when XSAVE is not clear in policy

This fixes booting of old non-PV-OPS kernels which historically
looked for OSXSAVE instead of XSAVE bit in CPUID to check whether
XSAVE feature is enabled. If such a guest appears to be started on
an XSAVE enabled CPU and the feature is explicitly cleared in
policy, leaked OSXSAVE bit from Xen will lead to guest crash early in
boot.

Signed-off-by: Igor Druzhinin <igor.druzhinin@citrix.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>
master commit: 902888922e
master date: 2019-06-28 13:17:53 +0100

xen/arch/x86/cpuid.c

@@ -781,7 +781,8 @@ void guest_cpuid(const struct vcpu *v, uint32_t leaf,
              *    damage itself.
              *
              * - Enlightened CPUID or CPUID faulting available:
-             *    Xen can fully control what is seen here.  Guest kernels need
+             *    Xen can fully control what is seen here.  When the guest has
+             *    been configured to have XSAVE available, guest kernels need
              *    to see the leaked OSXSAVE via the enlightened path, but
              *    guest userspace and the native is given architectural
              *    behaviour.
@@ -791,7 +792,8 @@ void guest_cpuid(const struct vcpu *v, uint32_t leaf,
              */
             /* OSXSAVE clear in policy.  Fast-forward CR4 back in. */
             if ( (v->arch.pv.ctrlreg[4] & X86_CR4_OSXSAVE) ||
-                 (regs->entry_vector == TRAP_invalid_op &&
+                 (p->basic.xsave &&
+                  regs->entry_vector == TRAP_invalid_op &&
                   guest_kernel_mode(v, regs) &&
                   (read_cr4() & X86_CR4_OSXSAVE)) )
                 res->c |= cpufeat_mask(X86_FEATURE_OSXSAVE);