KVM: selftests: Trust that MAXPHYADDR > memslot0 in vmx_apic_access_test

Use vm->max_gfn to compute the highest gpa in vmx_apic_access_test, and
blindly trust that the highest gfn/gpa will be well above the memory
carved out for memslot0. The existing check is beyond paranoid; KVM
doesn't support CPUs with host.MAXPHYADDR < 32, and the selftests are all
kinds of hosed if memslot0 overlaps the local xAPIC, which resides above
"lower" (below 4gb) DRAM.

Signed-off-by: Sean Christopherson <[e-mail]>
Signed-off-by: Paolo Bonzini <[e-mail]>

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

···
 int main(int argc, char *argv[])
 {
 	unsigned long apic_access_addr = ~0ul;
-	unsigned int paddr_width;
-	unsigned int vaddr_width;
 	vm_vaddr_t vmx_pages_gva;
 	unsigned long high_gpa;
 	struct vmx_pages *vmx;
···
 
 	vm = vm_create_with_one_vcpu(&vcpu, l1_guest_code);
 
-	kvm_get_cpu_address_width(&paddr_width, &vaddr_width);
-	high_gpa = (1ul << paddr_width) - getpagesize();
-	if ((unsigned long)DEFAULT_GUEST_PHY_PAGES * getpagesize() > high_gpa) {
-		print_skip("No unbacked physical page available");
-		exit(KSFT_SKIP);
-	}
+	high_gpa = (vm->max_gfn - 1) << vm->page_shift;
 
 	vmx = vcpu_alloc_vmx(vm, &vmx_pages_gva);
 	prepare_virtualize_apic_accesses(vmx, vm);