On RV32, updating the 64-bit stimecmp (or vstimecmp) CSR requires two
separate 32-bit writes. A race condition exists if the timer triggers
during these two writes.

The RISC-V Privileged Specification (e.g., Section 3.2.1 for mtimecmp)
recommends a specific 3-step sequence to avoid spurious interrupts
when updating 64-bit comparison registers on 32-bit systems:

1. Set the low-order bits (stimecmp) to all ones (ULONG_MAX).
2. Set the high-order bits (stimecmph) to the desired value.
3. Set the low-order bits (stimecmp) to the desired value.

Current implementation writes the LSB first without ensuring a future
value, which may lead to a transient state where the 64-bit comparison
is incorrectly evaluated as "expired" by the hardware. This results in
spurious timer interrupts.

This patch adopts the spec-recommended 3-step sequence to ensure the
intermediate 64-bit state is never smaller than the current time.

Fixes: ffef54ad4110 ("riscv: Add stimecmp save and restore")
Signed-off-by: Naohiko Shimizu <naohiko.shimizu@gmail.com>
---
 arch/riscv/kernel/suspend.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/arch/riscv/kernel/suspend.c b/arch/riscv/kernel/suspend.c
index 24b3f57d467f..aff93090c4ef 100644
--- a/arch/riscv/kernel/suspend.c
+++ b/arch/riscv/kernel/suspend.c
@@ -51,10 +51,11 @@ void suspend_restore_csrs(struct suspend_context *context)
 
 #ifdef CONFIG_MMU
 	if (riscv_has_extension_unlikely(RISCV_ISA_EXT_SSTC)) {
-		csr_write(CSR_STIMECMP, context->stimecmp);
 #if __riscv_xlen < 64
+		csr_write(CSR_STIMECMP, ULONG_MAX);
 		csr_write(CSR_STIMECMPH, context->stimecmph);
 #endif
+		csr_write(CSR_STIMECMP, context->stimecmp);
 	}
 
 	csr_write(CSR_SATP, context->satp);
-- 
2.39.5

On Sun, Jan 4, 2026 at 7:30 PM Naohiko Shimizu
<naohiko.shimizu@gmail.com> wrote:
>
> On RV32, updating the 64-bit stimecmp (or vstimecmp) CSR requires two
> separate 32-bit writes. A race condition exists if the timer triggers
> during these two writes.
>
> The RISC-V Privileged Specification (e.g., Section 3.2.1 for mtimecmp)
> recommends a specific 3-step sequence to avoid spurious interrupts
> when updating 64-bit comparison registers on 32-bit systems:
>
> 1. Set the low-order bits (stimecmp) to all ones (ULONG_MAX).
> 2. Set the high-order bits (stimecmph) to the desired value.
> 3. Set the low-order bits (stimecmp) to the desired value.
>
> Current implementation writes the LSB first without ensuring a future
> value, which may lead to a transient state where the 64-bit comparison
> is incorrectly evaluated as "expired" by the hardware. This results in
> spurious timer interrupts.
>
> This patch adopts the spec-recommended 3-step sequence to ensure the
> intermediate 64-bit state is never smaller than the current time.
>
> Fixes: ffef54ad4110 ("riscv: Add stimecmp save and restore")
> Signed-off-by: Naohiko Shimizu <naohiko.shimizu@gmail.com>

LGTM.

Reviewed-by: Anup Patel <anup@brainfault.org>

Regards,
Anup


> ---
>  arch/riscv/kernel/suspend.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
>
> diff --git a/arch/riscv/kernel/suspend.c b/arch/riscv/kernel/suspend.c
> index 24b3f57d467f..aff93090c4ef 100644
> --- a/arch/riscv/kernel/suspend.c
> +++ b/arch/riscv/kernel/suspend.c
> @@ -51,10 +51,11 @@ void suspend_restore_csrs(struct suspend_context *context)
>
>  #ifdef CONFIG_MMU
>         if (riscv_has_extension_unlikely(RISCV_ISA_EXT_SSTC)) {
> -               csr_write(CSR_STIMECMP, context->stimecmp);
>  #if __riscv_xlen < 64
> +               csr_write(CSR_STIMECMP, ULONG_MAX);
>                 csr_write(CSR_STIMECMPH, context->stimecmph);
>  #endif
> +               csr_write(CSR_STIMECMP, context->stimecmp);
>         }
>
>         csr_write(CSR_SATP, context->satp);
> --
> 2.39.5
>