[PATCH v6 15/20] xen/riscv: add minimal stuff to processor.h to build full Xen

[Oleksii Kurochko <oleksii.kurochko@xxxxxxxxx>]

The cpu_relax() function, introduced in this commit, is anticipated to
support _zihintpause by the CPU.

Signed-off-by: Oleksii Kurochko <oleksii.kurochko@xxxxxxxxx>
---
Changes in V6:
 - drop incorrect part in riscv/booting.txt and move the introduction of it to
   separate patch.
 - compiler check that __riscv_zihintpause exist was droped to separate patch.
 - minor fixes.
 - drop unnecessary comment.
 - update the commit message.
---
Changes in V5:
 - Code style fixes.
 - drop introduced TOOLCHAIN_HAS_ZIHINTPAUSE and use as-insn instead and use
   as-insn istead.
---
Changes in V4:
 - Change message -> subject in "Changes in V3"
 - Documentation about system requirement was added. In the future, it can be 
checked if the extension is supported
   by system __riscv_isa_extension_available() ( 
https://gitlab.com/xen-project/people/olkur/xen/-/commit/737998e89ed305eb92059300c374dfa53d2143fa
 )
 - update cpu_relax() function to check if __riscv_zihintpause is supported by 
a toolchain
 - add conditional _zihintpause to -march if it is supported by a toolchain
Changes in V3:
 - update the commit subject
 - rename get_processor_id to smp_processor_id
 - code style fixes
 - update the cpu_relax instruction: use pause instruction instead of div %0, 
%0, zero
---
Changes in V2:
 - Nothing changed. Only rebase.
---
 xen/arch/riscv/include/asm/processor.h | 20 ++++++++++++++++++++
 1 file changed, 20 insertions(+)

diff --git a/xen/arch/riscv/include/asm/processor.h 
b/xen/arch/riscv/include/asm/processor.h
index 6db681d805..6846151717 100644
--- a/xen/arch/riscv/include/asm/processor.h
+++ b/xen/arch/riscv/include/asm/processor.h
@@ -12,6 +12,9 @@
 
 #ifndef __ASSEMBLY__
 
+/* TODO: need to be implemeted */
+#define smp_processor_id() 0
+
 /* On stack VCPU state */
 struct cpu_user_regs
 {
@@ -53,6 +56,23 @@ struct cpu_user_regs
     unsigned long pregs;
 };
 
+/* TODO: need to implement */
+#define cpu_to_core(cpu)   0
+#define cpu_to_socket(cpu) 0
+
+static inline void cpu_relax(void)
+{
+#ifdef __riscv_zihintpause
+    /* Reduce instruction retirement. */
+    __asm__ __volatile__ ( "pause" );
+#else
+    /* Encoding of the pause instruction */
+    __asm__ __volatile__ ( ".insn 0x0100000F" );
+#endif
+
+    barrier();
+}
+
 static inline void wfi(void)
 {
     __asm__ __volatile__ ("wfi");
-- 
2.43.0