[PATCH] target/riscv: Support SW update of PTE A/D bits and Ssptwad extension

[Jim Shu]

RISC-V priv spec v1.12 permits 2 PTE-update schemes of A/D-bit
(Access/Dirty bit): HW update or SW update. RISC-V profile defines the
extension name 'Ssptwad' for HW update to PTE A/D bits.
https://github.com/riscv/riscv-profiles/blob/main/profiles.adoc

Current QEMU RISC-V implements HW update scheme, so this commit
introduces SW update scheme to QEMU and uses the 'Ssptwad' extension
as the CPU option to select 2 PTE-update schemes. QEMU RISC-V CPU still
uses HW update scheme (ext_ssptwad=true) by default to keep the backward
compatibility.

SW update scheme implemention is based on priv spec v1.12:
"When a virtual page is accessed and the A bit is clear, or is written
and the D bit is clear, a page-fault exception (corresponding to the
original access type) is raised."

Signed-off-by: Jim Shu <jim.shu@sifive.com>
Reviewed-by: Frank Chang <frank.chang@sifive.com>
---
 target/riscv/cpu.c        | 2 ++
 target/riscv/cpu.h        | 1 +
 target/riscv/cpu_helper.c | 9 +++++++++
 3 files changed, 12 insertions(+)

diff --git a/target/riscv/cpu.c b/target/riscv/cpu.c
index 1bb3973806..1d38c1c1d2 100644
--- a/target/riscv/cpu.c
+++ b/target/riscv/cpu.c
@@ -857,6 +857,7 @@ static void riscv_cpu_init(Object *obj)
 
     cpu->cfg.ext_ifencei = true;
     cpu->cfg.ext_icsr = true;
+    cpu->cfg.ext_ssptwad = true;
     cpu->cfg.mmu = true;
     cpu->cfg.pmp = true;
 
@@ -900,6 +901,7 @@ static Property riscv_cpu_extensions[] = {
     DEFINE_PROP_BOOL("svinval", RISCVCPU, cfg.ext_svinval, false),
     DEFINE_PROP_BOOL("svnapot", RISCVCPU, cfg.ext_svnapot, false),
     DEFINE_PROP_BOOL("svpbmt", RISCVCPU, cfg.ext_svpbmt, false),
+    DEFINE_PROP_BOOL("ssptwad", RISCVCPU, cfg.ext_ssptwad, true),
 
     DEFINE_PROP_BOOL("zba", RISCVCPU, cfg.ext_zba, true),
     DEFINE_PROP_BOOL("zbb", RISCVCPU, cfg.ext_zbb, true),
diff --git a/target/riscv/cpu.h b/target/riscv/cpu.h
index 5c7acc055a..2eee59af98 100644
--- a/target/riscv/cpu.h
+++ b/target/riscv/cpu.h
@@ -433,6 +433,7 @@ struct RISCVCPUConfig {
     bool ext_zve32f;
     bool ext_zve64f;
     bool ext_zmmul;
+    bool ext_ssptwad;
     bool rvv_ta_all_1s;
 
     uint32_t mvendorid;
diff --git a/target/riscv/cpu_helper.c b/target/riscv/cpu_helper.c
index 59b3680b1b..a8607c0d7b 100644
--- a/target/riscv/cpu_helper.c
+++ b/target/riscv/cpu_helper.c
@@ -981,6 +981,15 @@ restart:
 
             /* Page table updates need to be atomic with MTTCG enabled */
             if (updated_pte != pte) {
+                if (!cpu->cfg.ext_ssptwad) {
+                    /*
+                     * If A/D bits are managed by SW, HW just raises the
+                     * page fault exception corresponding to the original
+                     * access type when A/D bits need to be updated.
+                     */
+                    return TRANSLATE_FAIL;
+                }
+
                 /*
                  * - if accessed or dirty bits need updating, and the PTE is
                  *   in RAM, then we do so atomically with a compare and swap.
-- 
2.17.1