KVM: x86/xen: Use hva_t for holding hypercall page address

Use hva_t, a.k.a. unsigned long, for the local variable that holds the
hypercall page address.  On 32-bit KVM, gcc complains about using a u64
due to the implicit cast from a 64-bit value to a 32-bit pointer.

  arch/x86/kvm/xen.c: In function ‘kvm_xen_write_hypercall_page’:
  arch/x86/kvm/xen.c:300:22: error: cast to pointer from integer of
                             different size [-Werror=int-to-pointer-cast]
  300 |   page = memdup_user((u8 __user *)blob_addr, PAGE_SIZE);

Cc: Joao Martins <joao.m.martins@oracle.com>
Cc: David Woodhouse <dwmw@amazon.co.uk>
Fixes: 23200b7a ("KVM: x86/xen: intercept xen hypercalls if enabled")
Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20210208201502.1239867-1-seanjc@google.com>
Acked-by: David Woodhouse <dwmw@amazon.co.uk>
Reviewed-by: Joao Martins <joao.m.martins@oracle.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/xen.c

@@ -284,8 +284,12 @@ int kvm_xen_write_hypercall_page(struct kvm_vcpu *vcpu, u64 data)
 				return 1;
 		}
 	} else {
-		u64 blob_addr = lm ? kvm->arch.xen_hvm_config.blob_addr_64
-				   : kvm->arch.xen_hvm_config.blob_addr_32;
+		/*
+		 * Note, truncation is a non-issue as 'lm' is guaranteed to be
+		 * false for a 32-bit kernel, i.e. when hva_t is only 4 bytes.
+		 */
+		hva_t blob_addr = lm ? kvm->arch.xen_hvm_config.blob_addr_64
+				     : kvm->arch.xen_hvm_config.blob_addr_32;
 		u8 blob_size = lm ? kvm->arch.xen_hvm_config.blob_size_64
 				  : kvm->arch.xen_hvm_config.blob_size_32;
 		u8 *page;