xen/riscv: introduce extension support check by compiler

Currently, RISC-V requires two extensions: _zbb and _zihintpause.

This patch introduces a compiler check to check if these extensions
are supported.
Additionally, it introduces the riscv/booting.txt file, which contains
information about the extensions that should be supported by the platform.

In the future, a feature will be introduced to check whether an extension
is supported at runtime.
However, this feature requires functionality for parsing device tree
source (DTS), which is not yet available.

Signed-off-by: Oleksii Kurochko <oleksii.kurochko@gmail.com>
Acked-by: Jan Beulich <jbeulich@suse.com>

docs/misc/riscv/booting.txt

@@ -0,0 +1,16 @@
+System requirements
+===================
+
+The following extensions are expected to be supported by a system on which
+Xen is run:
+- Zbb:
+  RISC-V doesn't have a CLZ instruction in the base ISA.
+  As a consequence, __builtin_ffs() emits a library call to ffs() on GCC,
+  or a de Bruijn sequence on Clang.
+  Zbb extension adds a CLZ instruction, after which __builtin_ffs() emits
+  a very simple sequence.
+  The similar issue occurs with other __builtin_<bitop>, so it is needed to
+  provide a generic version of bitops in RISC-V bitops.h
+- Zihintpause:
+  On a system that doesn't have this extension, cpu_relax() should be
+  implemented properly.

xen/arch/riscv/arch.mk

@@ -3,16 +3,27 @@
 
 $(call cc-options-add,CFLAGS,CC,$(EMBEDDED_EXTRA_CFLAGS))
 
-CFLAGS-$(CONFIG_RISCV_64) += -mabi=lp64
+riscv-abi-$(CONFIG_RISCV_32) := -mabi=ilp32
+riscv-abi-$(CONFIG_RISCV_64) := -mabi=lp64
 
 riscv-march-$(CONFIG_RISCV_ISA_RV64G) := rv64g
 riscv-march-$(CONFIG_RISCV_ISA_C)       := $(riscv-march-y)c
 
+riscv-generic-flags := $(riscv-abi-y) -march=$(riscv-march-y)
+
+zbb := $(call as-insn,$(CC) $(riscv-generic-flags)_zbb,"",_zbb)
+zihintpause := $(call as-insn, \
+                      $(CC) $(riscv-generic-flags)_zihintpause,"pause",_zihintpause)
+
+extensions := $(zbb) $(zihintpause)
+
+extensions := $(subst $(space),,$(extensions))
+
 # Note that -mcmodel=medany is used so that Xen can be mapped
 # into the upper half _or_ the lower half of the address space.
 # -mcmodel=medlow would force Xen into the lower half.
 
-CFLAGS += -march=$(riscv-march-y) -mstrict-align -mcmodel=medany
+CFLAGS += $(riscv-generic-flags)$(extensions) -mstrict-align -mcmodel=medany
 
 # TODO: Drop override when more of the build is working
 override ALL_OBJS-y = arch/$(SRCARCH)/built_in.o