KVM: selftests: Drop manual XCR0 configuration from SEV smoke test

Now that CR4.OSXSAVE and XCR0 are setup by default, drop the manual
enabling from the SEV smoke test that validates FPU state can be
transferred into the VMSA.

In guest_code_xsave(), explicitly set the Requested-Feature Bitmask (RFBM)
to exactly XFEATURE_MASK_X87_AVX instead of relying on the host side of
things to enable only X87_AVX features in guest XCR0.  I.e. match the RFBM
for the host XSAVE.

Link: https://lore.kernel.org/r/20241003234337.273364-11-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

Author: sean-jc

tools/testing/selftests/kvm/x86_64/sev_smoke_test.c

@@ -41,8 +41,8 @@ static void guest_sev_code(void)
 /* Stash state passed via VMSA before any compiled code runs.  */
 extern void guest_code_xsave(void);
 asm("guest_code_xsave:\n"
-    "mov $-1, %eax\n"
-    "mov $-1, %edx\n"
+    "mov $" __stringify(XFEATURE_MASK_X87_AVX) ", %eax\n"
+    "xor %edx, %edx\n"
     "xsave (%rdi)\n"
     "jmp guest_sev_es_code");
 
@@ -70,12 +70,6 @@ static void test_sync_vmsa(uint32_t policy)
 
 	double x87val = M_PI;
 	struct kvm_xsave __attribute__((aligned(64))) xsave = { 0 };
-	struct kvm_sregs sregs;
-	struct kvm_xcrs xcrs = {
-		.nr_xcrs = 1,
-		.xcrs[0].xcr = 0,
-		.xcrs[0].value = XFEATURE_MASK_X87_AVX,
-	};
 
 	vm = vm_sev_create_with_one_vcpu(KVM_X86_SEV_ES_VM, guest_code_xsave, &vcpu);
 	gva = vm_vaddr_alloc_shared(vm, PAGE_SIZE, KVM_UTIL_MIN_VADDR,
@@ -84,11 +78,6 @@ static void test_sync_vmsa(uint32_t policy)
 
 	vcpu_args_set(vcpu, 1, gva);
 
-	vcpu_sregs_get(vcpu, &sregs);
-	sregs.cr4 |= X86_CR4_OSFXSR | X86_CR4_OSXSAVE;
-	vcpu_sregs_set(vcpu, &sregs);
-
-	vcpu_xcrs_set(vcpu, &xcrs);
 	asm("fninit\n"
 	    "vpcmpeqb %%ymm4, %%ymm4, %%ymm4\n"
 	    "fldl %3\n"