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Re: [PATCH 11/13] target/riscv: Switch context in exception return

From: LIU Zhiwei
Subject: Re: [PATCH 11/13] target/riscv: Switch context in exception return
Date: Tue, 9 Nov 2021 14:51:39 +0800
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.13.0

We only have to process the special CSRs(like vtype/misa) when xlen changes,  according to the explicitly  specified behavior about the CSR width change behavior. For normal CSRs, the default behavior in section 2.4 , CSR Width Modulation, is enough.

And if we split the vill out, we will never need this patch. So I will remove it next patch set.


On 2021/11/8 下午7:23, LIU Zhiwei wrote:

On 2021/11/2 上午12:43, Richard Henderson wrote:
On 11/1/21 6:01 AM, LIU Zhiwei wrote:
After excpetion return, we should give a xlen view of context in new
priveledge, including the general registers, pc, and CSRs.

Signed-off-by: LIU Zhiwei <zhiwei_liu@c-sky.com>
  target/riscv/helper.h                         |  1 +
  .../riscv/insn_trans/trans_privileged.c.inc   |  2 ++
  target/riscv/op_helper.c                      | 26 +++++++++++++++++++
  3 files changed, 29 insertions(+)

diff --git a/target/riscv/helper.h b/target/riscv/helper.h
index e198d43981..9b379d7232 100644
--- a/target/riscv/helper.h
+++ b/target/riscv/helper.h
@@ -71,6 +71,7 @@ DEF_HELPER_2(sret, tl, env, tl)
  DEF_HELPER_2(mret, tl, env, tl)
  DEF_HELPER_1(wfi, void, env)
  DEF_HELPER_1(tlb_flush, void, env)
+DEF_HELPER_1(switch_context_xl, void, env)
    /* Hypervisor functions */
diff --git a/target/riscv/insn_trans/trans_privileged.c.inc b/target/riscv/insn_trans/trans_privileged.c.inc
index 75c6ef80a6..6e39632f83 100644
--- a/target/riscv/insn_trans/trans_privileged.c.inc
+++ b/target/riscv/insn_trans/trans_privileged.c.inc
@@ -78,6 +78,7 @@ static bool trans_sret(DisasContext *ctx, arg_sret *a)
        if (has_ext(ctx, RVS)) {
          gen_helper_sret(cpu_pc, cpu_env, cpu_pc);
+        gen_helper_switch_context_xl(cpu_env);
          tcg_gen_exit_tb(NULL, 0); /* no chaining */
          ctx->base.is_jmp = DISAS_NORETURN;
      } else {
@@ -94,6 +95,7 @@ static bool trans_mret(DisasContext *ctx, arg_mret *a)
      tcg_gen_movi_tl(cpu_pc, ctx->base.pc_next);
      gen_helper_mret(cpu_pc, cpu_env, cpu_pc);
+    gen_helper_switch_context_xl(cpu_env);
      tcg_gen_exit_tb(NULL, 0); /* no chaining */
      ctx->base.is_jmp = DISAS_NORETURN;
      return true;
diff --git a/target/riscv/op_helper.c b/target/riscv/op_helper.c
index ee7c24efe7..20cf8ad883 100644
--- a/target/riscv/op_helper.c
+++ b/target/riscv/op_helper.c
@@ -70,6 +70,32 @@ target_ulong helper_csrrw(CPURISCVState *env, int csr,
    #ifndef CONFIG_USER_ONLY
+void helper_switch_context_xl(CPURISCVState *env)
+    RISCVMXL xl = cpu_get_xl(env);
+    if (xl == env->misa_mxl_max) {
+        return;
+    }
+    assert(xl < env->misa_mxl_max);
+    switch (xl) {
+    case MXL_RV32:
+        for (int i = 1; i < 32; i++) {
+            env->gpr[i] = (int32_t)env->gpr[i];
+        }

I think this is wrong.  As I read the spec, the new context ignores high bits and writes sign-extended values, but registers that are not written should not be changed.
I think so.

I think that a unit test with SXLEN == 64 and UXLEN == 32, where the U-mode program executes only the ECALL instruction, should leave the high 32 bits of all gprs unchanged on re-entry to S-mode.
Re-entry to U-mode?  I think you are right.  But currently I don't have a hardware has implemented the UXL32.

+        env->pc = (int32_t)env->pc;

I think this will happen naturally via patch 3.
Maybe we have to sign extend here as clarified in other reply.

+        /*
+         * For the read-only bits of the previous-width CSR, the bits at the
+         * same positions in the temporary register are set to zeros.
+         */
+        if ((env->priv == PRV_U) && (env->misa_ext & RVV)) {
+            env->vl = 0;
+            env->vtype = 0;
+        }

I don't understand this.  The return from the S-mode interrupt handler to the U-mode program should preserve the U-mode VTYPE.

According to the privileged architecture specification,  section 2.5,  if the width of a CSR is changed, the read-only bits of the previous-width CSR are zeroed. More general,  there is an algorithm how to calculate the new CSR value from the previous CSR.

I am not sure which is the right time  to do this context switch. The write time of  UXL field , or return to the U-mode?



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