[PATCH 1/3] target/riscv: zimop and zcmop extension for riscv

[Deepak Gupta]

`zimop` stands for `may be operations`. `zcmop` stands for compressed
`may be operations`. For some RISC-V CPU extension, once compiled into
the binary are part of generated code which can't be gated behind a probe
of whether an instruction set is supported or not. One such example is
`zicfiss` [1] extension where `shadow stack push` and `shadow stack pop
and check` will be part of every function body. Thus binaries compiled
with such extensions need to run in following scenarios

    - On machines where extension is present and enabled
    - On machines where extension is present and disabled
    - On machines where extension is not present/implemented.

`zimop` (for 32bit instructions) and `zcmop` (for compressed) were devised
and defined [2] to support such future (like zicfiss) CPU extensions
where zimops and zcmops provide a base non-faulting behavior for
codepoints that may claimed by future ISA extensions. Minimally, any
CPU implementation wanting to have binary compatibility with such
binaries only has to implement `zimop and zcmop`. Furthermore, this
allows per-task optin for software where user has the option to enable
the feature on per-task basis.

`zimop` are defined to write zero to `rd`. `zcmop` are defined to *not* write
to any register.

[1] - https://github.com/riscv/riscv-cfi/blob/main/src/cfi_backward.adoc
[2] - https://github.com/riscv/riscv-isa-manual/blob/main/src/zimop.adoc

Signed-off-by: Deepak Gupta <debug@rivosinc.com>
---
 target/riscv/cpu.c     | 2 ++
 target/riscv/cpu_cfg.h | 1 +
 2 files changed, 3 insertions(+)

diff --git a/target/riscv/cpu.c b/target/riscv/cpu.c
index eb1a2e7d6d..3caf8553d1 100644
--- a/target/riscv/cpu.c
+++ b/target/riscv/cpu.c
@@ -113,6 +113,7 @@ const RISCVIsaExtData isa_edata_arr[] = {
     ISA_EXT_DATA_ENTRY(zihintntl, PRIV_VERSION_1_10_0, ext_zihintntl),
     ISA_EXT_DATA_ENTRY(zihintpause, PRIV_VERSION_1_10_0, ext_zihintpause),
     ISA_EXT_DATA_ENTRY(zihpm, PRIV_VERSION_1_12_0, ext_zihpm),
+    ISA_EXT_DATA_ENTRY(zimops, PRIV_VERSION_1_12_0, ext_zimops),
     ISA_EXT_DATA_ENTRY(zmmul, PRIV_VERSION_1_12_0, ext_zmmul),
     ISA_EXT_DATA_ENTRY(za64rs, PRIV_VERSION_1_12_0, has_priv_1_11),
     ISA_EXT_DATA_ENTRY(zaamo, PRIV_VERSION_1_12_0, ext_zaamo),
@@ -2273,6 +2274,7 @@ static Property riscv_cpu_properties[] = {
      * it with -x and default to 'false'.
      */
     DEFINE_PROP_BOOL("x-misa-w", RISCVCPU, cfg.misa_w, false),
+    DEFINE_PROP_BOOL("zimops", RISCVCPU, cfg.ext_zimops, true),
     DEFINE_PROP_END_OF_LIST(),
 };
 
diff --git a/target/riscv/cpu_cfg.h b/target/riscv/cpu_cfg.h
index cb750154bd..5c42ff8cdf 100644
--- a/target/riscv/cpu_cfg.h
+++ b/target/riscv/cpu_cfg.h
@@ -124,6 +124,7 @@ struct RISCVCPUConfig {
     uint32_t mvendorid;
     uint64_t marchid;
     uint64_t mimpid;
+    bool ext_zimops;
 
     /* Named features  */
     bool ext_svade;
-- 
2.34.1