Re: [RFC PATCH 5/7] riscv: add double trap driver

[Deepak Gupta <debug@rivosinc.com>]

On Tue, May 14, 2024 at 10:06:31AM +0200, Clément Léger wrote:
>
>
>On 27/04/2024 01:59, Deepak Gupta wrote:
>> On Thu, Apr 18, 2024 at 04:26:44PM +0200, Clément Léger wrote:
>>> Add a small driver to request double trap enabling as well as
>>> registering a SSE handler for double trap. This will also be used by KVM
>>> SBI FWFT extension support to detect if it is possible to enable double
>>> trap in VS-mode.
>>>
>>> Signed-off-by: Clément Léger <cleger@rivosinc.com>
>>> ---
>>> arch/riscv/include/asm/sbi.h    |  1 +
>>> drivers/firmware/Kconfig        |  7 +++
>>> drivers/firmware/Makefile       |  1 +
>>> drivers/firmware/riscv_dbltrp.c | 95 +++++++++++++++++++++++++++++++++
>>> include/linux/riscv_dbltrp.h    | 19 +++++++
>>> 5 files changed, 123 insertions(+)
>>> create mode 100644 drivers/firmware/riscv_dbltrp.c
>>> create mode 100644 include/linux/riscv_dbltrp.h
>>>
>>> diff --git a/arch/riscv/include/asm/sbi.h b/arch/riscv/include/asm/sbi.h
>>> index 744aa1796c92..9cd4ca66487c 100644
>>> --- a/arch/riscv/include/asm/sbi.h
>>> +++ b/arch/riscv/include/asm/sbi.h
>>> @@ -314,6 +314,7 @@ enum sbi_sse_attr_id {
>>> #define SBI_SSE_ATTR_INTERRUPTED_FLAGS_SPIE    (1 << 2)
>>>
>>> #define SBI_SSE_EVENT_LOCAL_RAS        0x00000000
>>> +#define SBI_SSE_EVENT_LOCAL_DOUBLE_TRAP    0x00000001
>>> #define SBI_SSE_EVENT_GLOBAL_RAS    0x00008000
>>> #define SBI_SSE_EVENT_LOCAL_PMU        0x00010000
>>> #define SBI_SSE_EVENT_LOCAL_SOFTWARE    0xffff0000
>>> diff --git a/drivers/firmware/Kconfig b/drivers/firmware/Kconfig
>>> index 59f611288807..a037f6e89942 100644
>>> --- a/drivers/firmware/Kconfig
>>> +++ b/drivers/firmware/Kconfig
>>> @@ -197,6 +197,13 @@ config RISCV_SSE_TEST
>>>       Select if you want to enable SSE extension testing at boot time.
>>>       This will run a series of test which verifies SSE sanity.
>>>
>>> +config RISCV_DBLTRP
>>> +    bool "Enable Double trap handling"
>>> +    depends on RISCV_SSE && RISCV_SBI
>>> +    default n
>>> +    help
>>> +      Select if you want to enable SSE double trap handler.
>>> +
>>> config SYSFB
>>>     bool
>>>     select BOOT_VESA_SUPPORT
>>> diff --git a/drivers/firmware/Makefile b/drivers/firmware/Makefile
>>> index fb7b0c08c56d..ad67a1738c0f 100644
>>> --- a/drivers/firmware/Makefile
>>> +++ b/drivers/firmware/Makefile
>>> @@ -18,6 +18,7 @@ obj-$(CONFIG_RASPBERRYPI_FIRMWARE) += raspberrypi.o
>>> obj-$(CONFIG_FW_CFG_SYSFS)    += qemu_fw_cfg.o
>>> obj-$(CONFIG_RISCV_SSE)        += riscv_sse.o
>>> obj-$(CONFIG_RISCV_SSE_TEST)    += riscv_sse_test.o
>>> +obj-$(CONFIG_RISCV_DBLTRP)    += riscv_dbltrp.o
>>> obj-$(CONFIG_SYSFB)        += sysfb.o
>>> obj-$(CONFIG_SYSFB_SIMPLEFB)    += sysfb_simplefb.o
>>> obj-$(CONFIG_TI_SCI_PROTOCOL)    += ti_sci.o
>>> diff --git a/drivers/firmware/riscv_dbltrp.c
>>> b/drivers/firmware/riscv_dbltrp.c
>>> new file mode 100644
>>> index 000000000000..72f9a067e87a
>>> --- /dev/null
>>> +++ b/drivers/firmware/riscv_dbltrp.c
>>> @@ -0,0 +1,95 @@
>>> +// SPDX-License-Identifier: GPL-2.0-only
>>> +/*
>>> + * Copyright (C) 2023 Rivos Inc.
>>> + */
>>
>> nit: fix copyright year
>>> +
>>> +#define pr_fmt(fmt) "riscv-dbltrp: " fmt
>>> +
>>> +#include <linux/cpu.h>
>>> +#include <linux/init.h>
>>> +#include <linux/riscv_dbltrp.h>
>>> +#include <linux/riscv_sse.h>
>>> +
>>> +#include <asm/sbi.h>
>>> +
>>> +static bool double_trap_enabled;
>>> +
>>> +static int riscv_sse_dbltrp_handle(uint32_t evt, void *arg,
>>> +                   struct pt_regs *regs)
>>> +{
>>> +    __show_regs(regs);
>>> +    panic("Double trap !\n");
>>> +
>>> +    return 0;
>> Curious:
>> Does panic return?
>> What's the point of returning from here?
>
>Hi Deepak,
>
>No, panic() does not return and indeed, the "return 0" is useless. It's
>a leftover of a previous implementation without panic in order to keep
>GCC mouth shut ;).
>
>>
>>> +}
>>> +
>>> +struct cpu_dbltrp_data {
>>> +    int error;
>>> +};
>>> +
>>> +static void
>>> +sbi_cpu_enable_double_trap(void *data)
>>> +{
>>> +    struct sbiret ret;
>>> +    struct cpu_dbltrp_data *cdd = data;
>>> +
>>> +    ret = sbi_ecall(SBI_EXT_FWFT, SBI_EXT_FWFT_SET,
>>> +            SBI_FWFT_DOUBLE_TRAP_ENABLE, 1, 0, 0, 0, 0);
>>> +
>>> +    if (ret.error) {
>>> +        cdd->error = 1;
>>> +        pr_err("Failed to enable double trap on cpu %d\n",
>>> smp_processor_id());
>>> +    }
>>> +}
>>> +
>>> +static int sbi_enable_double_trap(void)
>>> +{
>>> +    struct cpu_dbltrp_data cdd = {0};
>>> +
>>> +    on_each_cpu(sbi_cpu_enable_double_trap, &cdd, 1);
>>> +    if (cdd.error)
>>> +        return -1;
>>
>> There is a bug here. If `sbi_cpu_enable_double_trap` failed on all cpus
>> but last cpu.
>> Then cdd.error would not record error and will be reflect as if double
>> trap was enabled.
>
>cdd.error is only written in case of error by the per-cpu callbacks, so
>it is only set if enabled failed. Is there something I'm missing ?

No. Sorry I missed that detail. lgtm.

>
>Thanks,
>
>Clément
>
>>
>> Its less likely to happen that FW would return success for one cpu and
>> fail for others.
>> But there is non-zero probablity here.
>>
>