Re: [PATCH v12 02/11] riscv: add support for SBI Supervisor Software Events extension

[Zhanpeng Zhang]

Hi,

On 10/29/26 9:48 PM, Himanshu Chauhan wrote:
> Hi,
>
> On Thu, Oct 29, 2026 at 03:48:44PM +0800, Zhanpeng Zhang wrote:
>> From: Clément Léger <cleger@xxxxxxxxxxxx>
>>
>> The SBI SSE extension allows firmware to notify supervisor software of
>> events that must be delivered independently of normal S-mode interrupts.
>> Firmware saves the minimal state required to enter the supervisor handler,
>> and Linux builds the synthetic handler context around it.
>>
>> SSE can arrive while Linux is already in an exception entry path. At that
>> point sscratch and tp may be in the middle of the normal trap-entry
>> exchange, so they cannot always identify current. Store current in a
>> per-CPU slot and use the hart ID passed by firmware to recover it.
>>
>> Give each event, including each CPU instance of a local event, a dedicated
>> stack and shadow call stack. Synchronize vmapped stack ranges before
>> unmasking events so that the handler cannot take a vmalloc fault while
>> running in an NMI-like context.
>>
>> The handler is a synthetic supervisor episode, but completion must resume
>> the context interrupted by the SSE. Preserve stvec and, when the hypervisor
>> extension is present, hstatus across the handler. Read the interrupted a6
>> and a7 values from the SSE attributes, construct pt_regs for the
>> interrupted context, and write back any changes made by the handler.
>>
>> Nested exceptions on the SSE event stack temporarily replace both
>> TASK_TI_KERNEL_SP and TASK_TI_USER_SP. Preserve their original values
>> across the handler and restore them before completing the event, so a
>> nested exception cannot leave the interrupted task referring to the event
>> stack.
>>
>> On RV64 with an MMU, early trap entry also uses thread_info.a0/a1/a2
>> as scratch slots before saving the full register frame. An SSE handler
>> can interrupt that window and take a nested fault which overwrites the
>> same slots. Preserve them in already-saved callee-saved registers and
>> restore them before resuming the interrupted entry path.
>>
>> Disable preemption while initializing the per-CPU entry-task slot.
>> CPU hotplug callbacks run in a preemptible thread, and this write
>> happens before the guards around the local SBI operations.
>>
>> Compile the stack TLB helper only for VMAP_STACK or SHADOW_CALL_STACK,
>> its only users, to avoid an unused-function warning in other builds.
>>
>> Keep an explicit EVENT_REGISTER not-supported result distinct from other
>> firmware failures. This lets clients select another delivery mechanism
>> only when firmware has positively rejected the requested event.
>>
>> Link: https://lore.kernel.org/r/as79MHtNYd-VbUug@xxxxxxxxxxxxxxxxxxxxxxxxxxx/
>> Reported-by: kernel test robot <lkp@xxxxxxxxx>
>> Closes: https://lore.kernel.org/oe-kbuild-all/202610020009.P3ILnDi1-lkp@xxxxxxxxx/
>> Signed-off-by: Clément Léger <cleger@xxxxxxxxxxxx>
>> Co-developed-by: Himanshu Chauhan <himanshu.chauhan@xxxxxxxxxxxxxxxx>
>> Signed-off-by: Himanshu Chauhan <himanshu.chauhan@xxxxxxxxxxxxxxxx>
>> Co-developed-by: Zhanpeng Zhang <zhangzhanpeng.jasper@xxxxxxxxxxxxx>
>> Signed-off-by: Zhanpeng Zhang <zhangzhanpeng.jasper@xxxxxxxxxxxxx>
>> ---
>>   arch/riscv/include/asm/asm.h         |  14 +-
>>   arch/riscv/include/asm/scs.h         |   7 +
>>   arch/riscv/include/asm/sse.h         |  82 +++++++++
>>   arch/riscv/include/asm/thread_info.h |   1 +
>>   arch/riscv/kernel/Makefile           |   1 +
>>   arch/riscv/kernel/asm-offsets.c      |  14 ++
>>   arch/riscv/kernel/entry.S            |  14 ++
>>   arch/riscv/kernel/sbi_sse.c          | 251 +++++++++++++++++++++++++++
>>   arch/riscv/kernel/sbi_sse_entry.S    | 237 +++++++++++++++++++++++++
>>   9 files changed, 618 insertions(+), 3 deletions(-)
>>   create mode 100644 arch/riscv/include/asm/sse.h
>>   create mode 100644 arch/riscv/kernel/sbi_sse.c
>>   create mode 100644 arch/riscv/kernel/sbi_sse_entry.S
>>
>> diff --git a/arch/riscv/include/asm/asm.h b/arch/riscv/include/asm/asm.h
>> index b8bf842d4c1..e1196caea02 100644
>> --- a/arch/riscv/include/asm/asm.h
>> +++ b/arch/riscv/include/asm/asm.h
>> @@ -91,16 +91,24 @@
>>   .endm
>>
>>   #ifdef CONFIG_SMP
>> -.macro asm_per_cpu dst sym tmp
>> -	lw    \tmp, TASK_TI_CPU_NUM(tp)
>> -	slli  \tmp, \tmp, RISCV_LGPTR
>> +.macro asm_per_cpu_with_cpu dst sym tmp cpu
>> +	slli  \tmp, \cpu, RISCV_LGPTR
>>   	la    \dst, __per_cpu_offset
>>   	add   \dst, \dst, \tmp
>>   	REG_L \tmp, 0(\dst)
>>   	la    \dst, \sym
>>   	add   \dst, \dst, \tmp
>>   .endm
>> +
>> +.macro asm_per_cpu dst sym tmp
>> +	lw \tmp, TASK_TI_CPU_NUM(tp)
>> +	asm_per_cpu_with_cpu \dst \sym \tmp \tmp
>> +.endm
>>   #else /* CONFIG_SMP */
>> +.macro asm_per_cpu_with_cpu dst sym tmp cpu
>> +	la    \dst, \sym
>> +.endm
>> +
>>   .macro asm_per_cpu dst sym tmp
>>   	la    \dst, \sym
>>   .endm
>> diff --git a/arch/riscv/include/asm/scs.h b/arch/riscv/include/asm/scs.h
>> index 023a412fe38..0d70a35bc01 100644
>> --- a/arch/riscv/include/asm/scs.h
>> +++ b/arch/riscv/include/asm/scs.h
>
> This patch doesn't apply on v7.3-rc7. Did you rebase on it?
> b4 shazam failed at this patch in scs.h file.
>
> Please check.
>
> Regards
> Himanshu
>

V12 uses a5ae1bf65cee, the same base as Anup's v11 and still the current
riscv/for-next tip, as Paul requested.

rc7 has an additional scs.h change that causes this conflict. Could you
please apply the series on a5ae1bf65cee instead?

>> [...]
Thanks,
Zhanpeng