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Message-ID: <8a9952bf-67ff-4ab0-9e44-e2c2f2a68076@rivosinc.com>
Date: Thu, 7 Dec 2023 10:23:20 +0100
From: Clément Léger <cleger@...osinc.com>
To: Xu Lu <luxu.kernel@...edance.com>
Cc: Paul Walmsley <paul.walmsley@...ive.com>,
Palmer Dabbelt <palmer@...belt.com>,
Albert Ou <aou@...s.berkeley.edu>,
Atish Patra <atishp@...shpatra.org>,
Anup Patel <apatel@...tanamicro.com>,
linux-riscv@...ts.infradead.org, linux-kernel@...r.kernel.org,
linux-arm-kernel@...ts.infradead.org,
Himanshu Chauhan <hchauhan@...tanamicro.com>
Subject: Re: [External] [RFC PATCH 0/3] riscv: add support for SBI Supervisor
Software Events
On 07/12/2023 10:09, Xu Lu wrote:
> Pardon. It seems that the code in opensbi[1] is not complete for PMU NMI now.
> For example, the pmu ovf irq is still delegated to supervisor mode and
> thus can not really play a role as NMI. And neither the kernel nor
> opensbi will inject a pmu event.
>
> To complete the work, we think maybe 'enable_cb' and 'disable_cb'
> functions can be supplied for sbi_sse_cb_ops.
> When sbi_sse_enable is called to enable pmu event, the enable_cb will
> be called to revoke the delegation of pmu ovf irq and enable this irq
> in CSR_MIE.
Hi Xu,
Indeed, this part has been developed but was left out for the RFC. But
your understanding is correct.
> When pmu ovf irq occurs, kernel traps into opensbi and opensbi injects
> the event via sbi_sse_inject_event and eret back to kernel.
>
> Please point it out if we have any misunderstanding.
>
> By the way, how is SSE going now? We will appreciate it very much if
> we can participate in some development work in kernel or opensbi and
> be of any help.
The development is almost complete, Anup/Himanchu will send a new
revision of the spec addressing various comments and I'll resend the RFC
following that spec update.
Regards,
Clément
>
> Regards!
>
> Link: https://github.com/rivosinc/opensbi/tree/dev/cleger/sse [1]
>
> On Thu, Oct 26, 2023 at 10:31 PM Clément Léger <cleger@...osinc.com> wrote:
>>
>> The SBI Supervisor Software Events (SSE) extensions provides a mechanism
>> to inject software events from an SBI implementation to supervisor
>> software such that it preempts all other supervisor level traps and
>> interrupts [1].
>>
>> Various events are defined and can be send asynchronously to supervisor
>> software (RAS, PMU, DEBUG, Asynchronous page fault) from SBI as well
>> as platform specific events. Events can be either local (per-hart) or
>> global. Events can be nested on top of each other based on priority and
>> can interrupt the kernel at any time.
>>
>> First patch adds the SSE definitions. Second one adds support for SSE
>> itself. Implementation is split between arch specific code and generic
>> part (similarly to what is done for ARM SDEI). Finally, the last patch
>> add support fro SSE event in the SBI PMU driver. If the SSE event is
>> available from the SBI then, it will be used instead of the normal
>> interrupt.
>>
>> Amongst the specific points that needs to be handle is the interruption
>> at any point of the kernel execution and more specifically during
>> exception handling. Due to the fact that the exception entry
>> implementation uses the SCRATCH CSR as both the current task struct and
>> as the temporary register to switch the stack and save register, it is
>> difficult to reliably get the current task struct if we get interrupted
>> at this specific moment. A fixup-like mechanism allows to mark the
>> location of the current task struct depending on the entry level
>> (user/kernel) and the location. This is then used in the SSE assembly to
>> determine where is located the current task_struct.
>>
>> Contrary to pseudo NMI [2], SSE does not modifies the way interrupts are
>> handled and does not adds any overhead to existing code. Moreover, it
>> provides "true" NMI-like interrupts which can interrupt the kernel at
>> any time (even in exception handling). This is particularly crucial for
>> RAS errors which needs to be handled as fast as possible to avoid any
>> fault propagation. Additionally, SSE event handling is faster that the
>> standard IRQ handling path with almost half executed instruction (700 vs
>> 1590). Some complementary tests/perf measurements will be done.
>>
>> For testing purpose, one can use the provided SBI implementation at [3].
>> This series also needs patch [4] to fix a bug in the PMU driver.
>>
>> Link: https://lists.riscv.org/g/tech-prs/message/515 [1]
>> Link: https://lore.kernel.org/lkml/20231023082911.23242-10-luxu.kernel@bytedance.com/T/ [2]
>> Link: https://github.com/rivosinc/opensbi/tree/dev/cleger/sse [3]
>> Link: https://lore.kernel.org/linux-arm-kernel/20231026084010.11888-1-alexghiti@rivosinc.com/ [4]
>>
>> ---
>>
>> Clément Léger (3):
>> riscv: add SBI SSE extension definitions
>> riscv: add support for SBI Supervisor Software Events extension
>> perf: RISC-V: add support for SSE event
>>
>> arch/riscv/include/asm/asm-prototypes.h | 5 +
>> arch/riscv/include/asm/sbi.h | 40 ++
>> arch/riscv/include/asm/sse.h | 94 +++++
>> arch/riscv/kernel/Makefile | 1 +
>> arch/riscv/kernel/asm-offsets.c | 17 +
>> arch/riscv/kernel/entry.S | 156 ++++++++
>> arch/riscv/kernel/sbi.c | 4 +
>> arch/riscv/kernel/sse.c | 97 +++++
>> arch/riscv/kernel/stacktrace.c | 13 +
>> arch/riscv/kernel/vmlinux.lds.S | 6 +
>> drivers/firmware/Kconfig | 10 +
>> drivers/firmware/Makefile | 1 +
>> drivers/firmware/riscv_sse.c | 496 ++++++++++++++++++++++++
>> drivers/perf/riscv_pmu_sbi.c | 51 ++-
>> include/linux/riscv_sse.h | 56 +++
>> 15 files changed, 1038 insertions(+), 9 deletions(-)
>> create mode 100644 arch/riscv/include/asm/sse.h
>> create mode 100644 arch/riscv/kernel/sse.c
>> create mode 100644 drivers/firmware/riscv_sse.c
>> create mode 100644 include/linux/riscv_sse.h
>>
>> --
>> 2.42.0
>>
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