Re: [RFC PATCH v2 4/6] RISC-V: Implement prctl call to set/get the memory consistency model
The standardisation of Ssdtso is on-hold (probably permanently) in response to feedback received during the review process. Unless a strong consensus forms around the need to standardise for one vertical or the other, I don't see Ssdtso moving forward given the concerns regarding possible fragmentation for the ecosystem.
Best, Philipp.
On Tue, 6 Jan 2026 at 22:54, Hans Boehm hboehm@google.com wrote:
I'm not sure what the Sstdso status is.
I would also greatly prefer to not have such a dynamic switching facility generally available, for reasons very similar to those that Will Deacon gave in the earlier ARM discussion. If this is generally available, I expect people to use it to work around memory ordering bugs in one software component by switching the entire process to TSO mode, even on hardware with an unoptimized TSO implementation. This will impose pressure to optimize TSO everywhere. We will increase pressure to ship TSO libraries to take advantage of situations in which we're running in TSO mode. I think there is significant risk of splitting the ecosystem in this way, possibly with an eventual painful migration from RVWMO to TSO. IIRC, SPARC went through something vaguely similar in its early days.
Dealing with one memory model per platform is complicated enough; we don't want two.
If you do this somewhere under the covers to implement an x86 VM, OK. But please don't let apps switch memory models.
Hans
On Mon, Jan 5, 2026 at 11:46 PM Yangyu Chen cyy@cyyself.name wrote:
Hi Mullner,
Thanks for this work, although it has already lasted for about 2 years.
On 9/2/2024 14:40, Christoph Müllner wrote:
We can use the PR_{S,G}ET_MEMORY_CONSISTENCY_MODEL prctl calls to change the memory consistency model at run-time if we have Ssdtso. This patch registers RISCV_WMO and RISCV_TSO as valid arguments for these prctl calls and implements the glue code to switch between these.
Signed-off-by: Christoph Müllner christoph.muellner@vrull.eu
.../mm/dynamic-memory-consistency-model.rst | 12 +++- arch/riscv/include/asm/processor.h | 7 ++ arch/riscv/kernel/Makefile | 1 + arch/riscv/kernel/dtso.c | 67 +++++++++++++++++++ include/uapi/linux/prctl.h | 2 + 5 files changed, 88 insertions(+), 1 deletion(-) create mode 100644 arch/riscv/kernel/dtso.c
diff --git a/Documentation/mm/dynamic-memory-consistency-model.rst b/Documentation/mm/dynamic-memory-consistency-model.rst index 1fce855a1fad..c8188c174e27 100644 --- a/Documentation/mm/dynamic-memory-consistency-model.rst +++ b/Documentation/mm/dynamic-memory-consistency-model.rst @@ -73,4 +73,14 @@ Supported memory consistency models This section defines the memory consistency models which are supported by the prctl interface.
-<none> +RISC-V +------
+RISC-V uses RVWMO (RISC-V weak memory ordering) as default memory consistency +model. TSO (total store ordering) is another specified model and provides +additional ordering guarantees. Switching user-mode processes from RVWMO to TSO +is possible when the Ssdtso extension is available.
+* :c:macro:`PR_MEMORY_CONSISTENCY_MODEL_RISCV_WMO`: RISC-V weak memory ordering (default).
+* :c:macro:`PR_MEMORY_CONSISTENCY_MODEL_RISCV_TSO`: RISC-V total store ordering. diff --git a/arch/riscv/include/asm/processor.h b/arch/riscv/include/asm/processor.h index a8509cc31ab2..05e05fddc94d 100644 --- a/arch/riscv/include/asm/processor.h +++ b/arch/riscv/include/asm/processor.h @@ -184,6 +184,13 @@ extern int set_unalign_ctl(struct task_struct *tsk, unsigned int val); #define GET_UNALIGN_CTL(tsk, addr) get_unalign_ctl((tsk), (addr)) #define SET_UNALIGN_CTL(tsk, val) set_unalign_ctl((tsk), (val))
+#ifdef CONFIG_RISCV_ISA_SSDTSO +extern int dtso_set_memory_consistency_model(unsigned long arg); +extern int dtso_get_memory_consistency_model(void); +#define SET_MEMORY_CONSISTENCY_MODEL(arg) dtso_set_memory_consistency_model(arg) +#define GET_MEMORY_CONSISTENCY_MODEL() dtso_get_memory_consistency_model() +#endif /* CONIG_RISCV_ISA_SSDTSO */
#endif /* __ASSEMBLY__ */
#endif /* _ASM_RISCV_PROCESSOR_H */
diff --git a/arch/riscv/kernel/Makefile b/arch/riscv/kernel/Makefile index f71910718053..85f7291da498 100644 --- a/arch/riscv/kernel/Makefile +++ b/arch/riscv/kernel/Makefile @@ -65,6 +65,7 @@ obj-$(CONFIG_RISCV_MISALIGNED) += traps_misaligned.o obj-$(CONFIG_FPU) += fpu.o obj-$(CONFIG_RISCV_ISA_V) += vector.o obj-$(CONFIG_RISCV_ISA_V) += kernel_mode_vector.o +obj-$(CONFIG_RISCV_ISA_SSDTSO) += dtso.o obj-$(CONFIG_SMP) += smpboot.o obj-$(CONFIG_SMP) += smp.o obj-$(CONFIG_SMP) += cpu_ops.o diff --git a/arch/riscv/kernel/dtso.c b/arch/riscv/kernel/dtso.c new file mode 100644 index 000000000000..591d5f9de0f5 --- /dev/null +++ b/arch/riscv/kernel/dtso.c @@ -0,0 +1,67 @@ +/* SPDX-License-Identifier: GPL-2.0-or-later */ +/*
- Copyright (c) 2024 Christoph Muellner christoph.muellner@vrull.eu
- */
+#include <linux/cpu.h> +#include <linux/smp.h> +#include <linux/prctl.h>
+#include <asm/cpu.h> +#include <asm/dtso.h>
+#include <trace/events/ipi.h>
+int dtso_set_memory_consistency_model(unsigned long arg) +{
int cpu;unsigned long cur_model = get_memory_consistency_model(current);unsigned long new_model;switch (arg) {case PR_MEMORY_CONSISTENCY_MODEL_RISCV_WMO:new_model = RISCV_MEMORY_CONSISTENCY_MODEL_WMO;break;case PR_MEMORY_CONSISTENCY_MODEL_RISCV_TSO:new_model = RISCV_MEMORY_CONSISTENCY_MODEL_TSO;break;default:return -EINVAL;}/* No change requested. */if (cur_model == new_model)return 0;/* Enabling TSO only works if DTSO is available. */if (new_model == PR_MEMORY_CONSISTENCY_MODEL_RISCV_TSO && !has_dtso())return -EINVAL;/* Switching TSO->WMO is not allowed. */if (new_model == RISCV_MEMORY_CONSISTENCY_MODEL_WMO)return -EINVAL;/* Set the new model in the task struct. */set_memory_consitency_model(current, new_model);/** We need to reschedule all threads of the current process.* Let's do this by rescheduling all CPUs.* This is stricter than necessary, but since this call is* not expected to happen frequently the impact is low.*/for_each_cpu(cpu, cpu_online_mask)smp_send_reschedule(cpu);return 0;+}
+int dtso_get_memory_consistency_model(void) +{
unsigned long cur_model = get_memory_consistency_model(current);if (cur_model == RISCV_MEMORY_CONSISTENCY_MODEL_TSO)return PR_MEMORY_CONSISTENCY_MODEL_RISCV_TSO;return PR_MEMORY_CONSISTENCY_MODEL_RISCV_WMO;+} diff --git a/include/uapi/linux/prctl.h b/include/uapi/linux/prctl.h index 579662731eaa..20264bdc3092 100644 --- a/include/uapi/linux/prctl.h +++ b/include/uapi/linux/prctl.h @@ -308,5 +308,7 @@ struct prctl_mm_map {
#define PR_SET_MEMORY_CONSISTENCY_MODEL 71 #define PR_GET_MEMORY_CONSISTENCY_MODEL 72 +# define PR_MEMORY_CONSISTENCY_MODEL_RISCV_WMO 1 +# define PR_MEMORY_CONSISTENCY_MODEL_RISCV_TSO 2
Should we replace "PR_MEMORY_CONSISTENCY_MODEL_RISCV_TSO" with "PR_MEMORY_CONSISTENCY_MODEL_TSO", so that it can share the same key as Apple's TSO implementation [1]? RISC-V Ssdtso would make such prctl more likely to be accepted.
[1] https://lore.kernel.org/lkml/20240411-tso-v1-0-754f11abfbff@marcan.st/
Thanks, Yangyu Chen
#endif /* _LINUX_PRCTL_H */
linux-kselftest-mirror@lists.linaro.org
-
Philipp Tomsich