Revert "KVM: selftests: Fix nested SVM tests when built with clang"

Revert back to using memset() in generic_svm_setup() now that KVM
selftests override memset() and friends specifically to prevent the
compiler from generating fancy code and/or linking to the libc
implementation.

This reverts commit ed290e1.

Suggested-by: Jim Mattson <jmattson@google.com>
Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20220928233652.783504-8-seanjc@google.com>
Reviewed-by: Jim Mattson <jmattson@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

tools/testing/selftests/kvm/lib/x86_64/svm.c

@@ -60,18 +60,6 @@ static void vmcb_set_seg(struct vmcb_seg *seg, u16 selector,
 	seg->base = base;
 }
 
-/*
- * Avoid using memset to clear the vmcb, since libc may not be
- * available in L1 (and, even if it is, features that libc memset may
- * want to use, like AVX, may not be enabled).
- */
-static void clear_vmcb(struct vmcb *vmcb)
-{
-	int n = sizeof(*vmcb) / sizeof(u32);
-
-	asm volatile ("rep stosl" : "+c"(n), "+D"(vmcb) : "a"(0) : "memory");
-}
-
 void generic_svm_setup(struct svm_test_data *svm, void *guest_rip, void *guest_rsp)
 {
 	struct vmcb *vmcb = svm->vmcb;
@@ -88,7 +76,7 @@ void generic_svm_setup(struct svm_test_data *svm, void *guest_rip, void *guest_r
 	wrmsr(MSR_EFER, efer | EFER_SVME);
 	wrmsr(MSR_VM_HSAVE_PA, svm->save_area_gpa);
 
-	clear_vmcb(vmcb);
+	memset(vmcb, 0, sizeof(*vmcb));
 	asm volatile ("vmsave %0\n\t" : : "a" (vmcb_gpa) : "memory");
 	vmcb_set_seg(&save->es, get_es(), 0, -1U, data_seg_attr);
 	vmcb_set_seg(&save->cs, get_cs(), 0, -1U, code_seg_attr);