xen/riscv: make Zihintpause no longer a required extension

Zihintpause is in required_extensions[] so Xen panics when it is missing,
while it does not strictly require hardware support for it.

The PAUSE hint (Zihintpause extension) is encoded as FENCE W, 0
(0x0100000F). In accordance with the RISC-V Unprivileged ISA, HINTs are
encoded in the space of valid standard instructions. On platforms without
Zihintpause support, executing 0x0100000F is treated as a standard FENCE
with an empty successor set, which acts as a NOP and never generates an
illegal instruction trap.

Zihintpause is therefore purely an optimization hint. Drop it from
required_extensions[] so Xen can still boot on systems that don't advertise
it, and print a warning in that case.

Assisted-by: Claude:claude-opus-5
Signed-off-by: Baptiste Le Duc <baptiste.le-duc@vates.tech>
Reviewed-by: Oleksii Kurochko <oleksii.kurochko@gmail.com>

docs/misc/riscv/booting.txt

@@ -19,9 +19,6 @@ them is missing from the riscv,isa property of the CPU nodes:
   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.
 - Svpbmt:
   Mandatory to enable changing the memory attributes of a page.
   For platforms that do not support Svpbmt, it is necessary to introduce a

xen/arch/riscv/cpufeature.c

@@ -214,7 +214,6 @@ static const struct riscv_isa_ext_data __initconst required_extensions[] = {
     RISCV_ISA_EXT_DATA(h),
     RISCV_ISA_EXT_DATA(zicsr),
     RISCV_ISA_EXT_DATA(zifencei),
-    RISCV_ISA_EXT_DATA(zihintpause),
     RISCV_ISA_EXT_DATA(zbb),
     RISCV_ISA_EXT_DATA(svpbmt),
 };
@@ -664,6 +663,17 @@ void __init riscv_fill_hwcap(void)
         __set_bit(RISCV_ISA_EXT_sstc, riscv_isa);
     }
 
+    /*
+     * Zihintpause isn't mandatory: the encoding used by cpu_relax() is a
+     * HINT which executes as a no-op on hardware without the extension.
+     * Report it, as a platform may provide its own way to hint a spin-wait
+     * loop, which then has to be wired up in cpu_relax().
+     */
+    if ( !riscv_isa_extension_available(NULL, RISCV_ISA_EXT_zihintpause) )
+        printk(XENLOG_WARNING
+               "Zihintpause unavailable: cpu_relax() gives the CPU no hint; "
+               "wire up this platform's pause equivalent in cpu_relax()\n");
+
     for ( i = 0; i < req_extns_amount; i++ )
     {
         const struct riscv_isa_ext_data ext = required_extensions[i];