Re: [PATCH] target/riscv: Use TSO barriers instead of full barriers for Ztso

[Palmer Dabbelt]

On Tue, 13 Oct 2026 04:57:49 PDT (-0700), daniel.barb...@oss.qualcomm.com wrote:


On 10/2/2026 10:47 AM, Rick Richard wrote:
With Ztso, plain loads and stores follow RVTSO, but the barriers
around them use TCG_MO_ALL.  That also orders a store before a later
load, which TSO doesn't require, and is expensive: on x86 hosts it
adds a locked instruction around every load and store in a
multi-threaded guest.

Thanks.  I'm not quite sure why I had TCG_MO_ALL in there, I think maybe 
I just misread the TCG_MO_* vs TCG_BAR_* blurb?

Reviewed-by: Palmer Dabbelt <pal...@dabbelt.com>

The barrier after a load now only keeps later loads and stores
behind it, and the barrier before a store only keeps earlier loads
and stores ahead of it.  That is all TSO needs.


Fixes: 09c4e88759 ("RISC-V: Add support for Ztso")

Signed-off-by: Rick Richard <r...@sloservers.com>
---

Reviewed-by: Daniel Henrique Barboza <daniel.barb...@oss.qualcomm.com>

I found this while looking into why riscv64 guests were so much
slower than aarch64 ones under bsd-user on FreeBSD.  The default -cpu
max enables Ztso, and turning it off with ztso=false accounted for
most of the difference.

Maybe we shouldn't default to Ztso, or at least not default to Ztso on 
host systems that themselves aren't TSO?

On an x86-64 host the patch made xz almost four times faster and cut
the time of a clang compile by about 30%, the same as running with
ztso=false.  The output didn't change.

I also tested it on a Raspberry Pi 3B+, which is arm64 and doesn't
give TSO ordering in hardware, to make sure the weaker barriers still
hold up there.  Everything ran correctly, and xz was about 12% faster.

  target/riscv/tcg/insn_trans/trans_rvi.c.inc | 4 ++--
  target/riscv/tcg/insn_trans/trans_rvv.c.inc | 4 ++--
  2 files changed, 4 insertions(+), 4 deletions(-)

diff --git a/target/riscv/tcg/insn_trans/trans_rvi.c.inc 
b/target/riscv/tcg/insn_trans/trans_rvi.c.inc
index 40b4c9aa..d31e4295 100644
--- a/target/riscv/tcg/insn_trans/trans_rvi.c.inc
+++ b/target/riscv/tcg/insn_trans/trans_rvi.c.inc
@@ -424,7 +424,7 @@ static bool gen_load(DisasContext *ctx, arg_lb *a, MemOp 
memop)
      }

      if (ctx->ztso) {
-        tcg_gen_mb(TCG_MO_ALL | TCG_BAR_LDAQ);
+        tcg_gen_mb(TCG_MO_LD_LD | TCG_MO_LD_ST | TCG_BAR_LDAQ);
      }

      return out;
@@ -485,7 +485,7 @@ static bool gen_store_tl(DisasContext *ctx, arg_sb *a, 
MemOp memop)
      TCGv data = get_gpr(ctx, a->rs2, EXT_NONE);

      if (ctx->ztso) {
-        tcg_gen_mb(TCG_MO_ALL | TCG_BAR_STRL);
+        tcg_gen_mb(TCG_MO_LD_ST | TCG_MO_ST_ST | TCG_BAR_STRL);
      }

      tcg_gen_qemu_st_tl(data, addr, ctx->mem_idx, memop);
diff --git a/target/riscv/tcg/insn_trans/trans_rvv.c.inc 
b/target/riscv/tcg/insn_trans/trans_rvv.c.inc
index a22e2cae..8c21ab87 100644
--- a/target/riscv/tcg/insn_trans/trans_rvv.c.inc
+++ b/target/riscv/tcg/insn_trans/trans_rvv.c.inc
@@ -731,7 +731,7 @@ static bool ldst_us_trans(uint32_t vd, uint32_t rs1, 
uint32_t data,
       * around the loop.
       */
      if (is_store && s->ztso) {
-        tcg_gen_mb(TCG_MO_ALL | TCG_BAR_STRL);
+        tcg_gen_mb(TCG_MO_LD_ST | TCG_MO_ST_ST | TCG_BAR_STRL);
      }

      mark_vs_dirty(s);
@@ -739,7 +739,7 @@ static bool ldst_us_trans(uint32_t vd, uint32_t rs1, 
uint32_t data,
      fn(dest, mask, base, tcg_env, desc);

      if (!is_store && s->ztso) {
-        tcg_gen_mb(TCG_MO_ALL | TCG_BAR_LDAQ);
+        tcg_gen_mb(TCG_MO_LD_LD | TCG_MO_LD_ST | TCG_BAR_LDAQ);
      }

      finalize_rvv_inst(s);