[PATCH v2 4/6] xen/riscv: make Zihintpause no longer a required extension

[Baptiste Le Duc <baptiste.le-duc@xxxxxxxxxx>]

required_extensions[] panics at boot if Zihintpause is missing, but Xen
never actually depends on it: cpu_relax() only emits the "pause" hint when
the extension is implemented, otherwise it emits `0x0100000F`, a legally
valid FENCE instruction (`FENCE W, 0`) rather than a native NOP. FENCE is
guaranteed by the RISC-V base ISA, so it never raises an illegal
instruction fault. With an empty successor set, it enforces no
memory-ordering constraints and thus architecturally behaves as a NOP.

Drop it from required_extensions so hardware without Zihintpause
still boots.

Assisted-by: Claude:claude-opus-5
Signed-off-by: Baptiste Le Duc <baptiste.le-duc@xxxxxxxxxx>
---
Changes since v1:
- rewrite commit message.
---
 xen/arch/riscv/cpufeature.c | 1 -
 1 file changed, 1 deletion(-)

diff --git a/xen/arch/riscv/cpufeature.c b/xen/arch/riscv/cpufeature.c
index 986a6dec78..41bb1d2e80 100644
--- a/xen/arch/riscv/cpufeature.c
+++ b/xen/arch/riscv/cpufeature.c
@@ -156,7 +156,6 @@ 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),
 };
 

-- 
2.55.0