powerpc/kernel: Add ucall_norets() ultravisor call handler

The ultracalls (ucalls for short) allow the Secure Virtual Machines
(SVM)s and hypervisor to request services from the ultravisor such as
accessing a register or memory region that can only be accessed when
running in ultravisor-privileged mode.

This patch adds the ucall_norets() ultravisor call handler.

The specific service needed from an ucall is specified in register
R3 (the first parameter to the ucall). Other parameters to the
ucall, if any, are specified in registers R4 through R12.

Return value of all ucalls is in register R3. Other output values
from the ucall, if any, are returned in registers R4 through R12.

Each ucall returns specific error codes, applicable in the context
of the ucall. However, like with the PowerPC Architecture Platform
Reference (PAPR), if no specific error code is defined for a particular
situation, then the ucall will fallback to an erroneous
parameter-position based code. i.e U_PARAMETER, U_P2, U_P3 etc depending
on the ucall parameter that may have caused the error.

Every host kernel (powernv) needs to be able to do ucalls in case it
ends up being run in a machine with ultravisor enabled. Otherwise, the
kernel may crash early in boot trying to access ultravisor resources,
for instance, trying to set the partition table entry 0. Secure guests
also need to be able to do ucalls and its kernel may not have
CONFIG_PPC_POWERNV=y. For that reason, the ucall.S file is placed under
arch/powerpc/kernel.

If ultravisor is not enabled, the ucalls will be redirected to the
hypervisor which must handle/fail the call.

Thanks to inputs from Ram Pai and Michael Anderson.
Signed-off-by: Claudio Carvalho <cclaudio@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20190822034838.27876-3-cclaudio@linux.ibm.com

arch/powerpc/include/asm/asm-prototypes.h

@@ -15,6 +15,7 @@
 #include <asm/epapr_hcalls.h>
 #include <asm/dcr.h>
 #include <asm/mmu_context.h>
+#include <asm/ultravisor-api.h>
 
 #include <uapi/asm/ucontext.h>
 
@@ -34,6 +35,16 @@ extern struct static_key hcall_tracepoint_key;
 void __trace_hcall_entry(unsigned long opcode, unsigned long *args);
 void __trace_hcall_exit(long opcode, long retval, unsigned long *retbuf);
 
+/* Ultravisor */
+#ifdef CONFIG_PPC_POWERNV
+long ucall_norets(unsigned long opcode, ...);
+#else
+static inline long ucall_norets(unsigned long opcode, ...)
+{
+	return U_NOT_AVAILABLE;
+}
+#endif
+
 /* OPAL */
 int64_t __opal_call(int64_t a0, int64_t a1, int64_t a2, int64_t a3,
 		    int64_t a4, int64_t a5, int64_t a6, int64_t a7,

arch/powerpc/include/asm/ultravisor-api.h

+/* SPDX-License-Identifier: GPL-2.0 */
+/*
+ * Ultravisor API.
+ *
+ * Copyright 2019, IBM Corporation.
+ *
+ */
+#ifndef _ASM_POWERPC_ULTRAVISOR_API_H
+#define _ASM_POWERPC_ULTRAVISOR_API_H
+
+#include <asm/hvcall.h>
+
+/* Return codes */
+#define U_FUNCTION		H_FUNCTION
+#define U_NOT_AVAILABLE		H_NOT_AVAILABLE
+#define U_P2			H_P2
+#define U_P3			H_P3
+#define U_P4			H_P4
+#define U_P5			H_P5
+#define U_PARAMETER		H_PARAMETER
+#define U_SUCCESS		H_SUCCESS
+
+#endif /* _ASM_POWERPC_ULTRAVISOR_API_H */

arch/powerpc/kernel/Makefile

@@ -156,6 +156,7 @@ endif
 
 obj-$(CONFIG_EPAPR_PARAVIRT)	+= epapr_paravirt.o epapr_hcalls.o
 obj-$(CONFIG_KVM_GUEST)		+= kvm.o kvm_emul.o
+obj-$(CONFIG_PPC_POWERNV)	+= ucall.o
 
 # Disable GCOV, KCOV & sanitizers in odd or sensitive code
 GCOV_PROFILE_prom_init.o := n

arch/powerpc/kernel/ucall.S

+/* SPDX-License-Identifier: GPL-2.0 */
+/*
+ * Generic code to perform an ultravisor call.
+ *
+ * Copyright 2019, IBM Corporation.
+ *
+ */
+#include <asm/ppc_asm.h>
+#include <asm/export.h>
+
+_GLOBAL(ucall_norets)
+EXPORT_SYMBOL_GPL(ucall_norets)
+	sc	2	/* Invoke the ultravisor */
+	blr		/* Return r3 = status */