KVM: x86: Zero XSTATE components on INIT by iterating over supported features

Tweak the code a bit to facilitate resetting more xstate components in
the future, e.g., CET's xstate-managed MSRs.

No functional change intended.

Suggested-by: Sean Christopherson <seanjc@google.com>
Tested-by: Mathias Krause <minipli@grsecurity.net>
Tested-by: John Allen <john.allen@amd.com>
Signed-off-by: Chao Gao <chao.gao@intel.com>
Tested-by: Rick Edgecombe <rick.p.edgecombe@intel.com>
Link: https://lore.kernel.org/r/20250812025606.74625-6-chao.gao@intel.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

Author: gaochaointel

arch/x86/kvm/x86.c

@@ -12401,6 +12401,8 @@ void kvm_arch_vcpu_destroy(struct kvm_vcpu *vcpu)
 static void kvm_xstate_reset(struct kvm_vcpu *vcpu, bool init_event)
 {
 	struct fpstate *fpstate = vcpu->arch.guest_fpu.fpstate;
+	u64 xfeatures_mask;
+	int i;
 
 	/*
 	 * Guest FPU state is zero allocated and so doesn't need to be manually
@@ -12414,16 +12416,20 @@ static void kvm_xstate_reset(struct kvm_vcpu *vcpu, bool init_event)
 	 * are unchanged.  Currently, the only components that are zeroed and
 	 * supported by KVM are MPX related.
 	 */
-	if (!kvm_mpx_supported())
+	xfeatures_mask = (kvm_caps.supported_xcr0 | kvm_caps.supported_xss) &
+			 (XFEATURE_MASK_BNDREGS | XFEATURE_MASK_BNDCSR);
+	if (!xfeatures_mask)
 		return;
 
+	BUILD_BUG_ON(sizeof(xfeatures_mask) * BITS_PER_BYTE <= XFEATURE_MAX);
+
 	/*
 	 * All paths that lead to INIT are required to load the guest's FPU
 	 * state (because most paths are buried in KVM_RUN).
 	 */
 	kvm_put_guest_fpu(vcpu);
-	fpstate_clear_xstate_component(fpstate, XFEATURE_BNDREGS);
-	fpstate_clear_xstate_component(fpstate, XFEATURE_BNDCSR);
+	for_each_set_bit(i, (unsigned long *)&xfeatures_mask, XFEATURE_MAX)
+		fpstate_clear_xstate_component(fpstate, i);
 	kvm_load_guest_fpu(vcpu);
 }