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Date:   Sat, 26 Sep 2020 22:39:06 -0700 (PDT)
Subject:     Re: [PATCH v2] RISC-V: Check clint_time_val before use
In-Reply-To: <CAAhSdy1EeeNcyHbk-MN+auKX+mZmvF=W=S-UPuRhrqVBjch4ow@mail.gmail.com>
CC:     Damien Le Moal <Damien.LeMoal@wdc.com>,
        Paul Walmsley <paul.walmsley@sifive.com>,
        Anup Patel <Anup.Patel@wdc.com>, aou@eecs.berkeley.edu,
        linux-riscv@lists.infradead.org, Atish Patra <Atish.Patra@wdc.com>,
        Alistair Francis <Alistair.Francis@wdc.com>,
        linux-kernel@vger.kernel.org
From:   Palmer Dabbelt <palmerdabbelt@google.com>
To:     anup@brainfault.org
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On Sat, 26 Sep 2020 22:38:17 PDT (-0700), anup@brainfault.org wrote:
> On Sun, Sep 27, 2020 at 5:50 AM Palmer Dabbelt <palmerdabbelt@google.com> wrote:
>>
>> On Sat, 26 Sep 2020 03:31:29 PDT (-0700), Damien Le Moal wrote:
>> > On Sat, 2020-09-26 at 15:51 +0530, Anup Patel wrote:
>> >> The NoMMU kernel is broken for QEMU virt machine from Linux-5.9-rc6
>> >> because the get_cycles() and friends are called very early from
>> >> rand_initialize() before CLINT driver is probed. To fix this, we
>> >> should check clint_time_val before use in get_cycles() and friends.
>>
>> I don't think this is the right way to solve that problem, as we're essentially
>> just lying about the timer rather than informing the system we can't get
>> timer-based entropy right now.  MIPS is explicit about this, I don't see any
>> reason why we shouldn't be as well.
>>
>> Does this fix the boot issue (see below for the NULL)?  There's some other
>> random-related arch functions so this might not be quite the right way to do
>> it.
>>
>> diff --git a/arch/riscv/include/asm/timex.h b/arch/riscv/include/asm/timex.h
>> index 7f659dda0032..7e39b0068932 100644
>> --- a/arch/riscv/include/asm/timex.h
>> +++ b/arch/riscv/include/asm/timex.h
>> @@ -33,6 +33,18 @@ static inline u32 get_cycles_hi(void)
>>  #define get_cycles_hi get_cycles_hi
>>  #endif /* CONFIG_64BIT */
>>
>> +/*
>> + * Much like MIPS, we may not have a viable counter to use at an early point in
>> + * the boot process.  Unfortunately we don't have a fallback, so instead we
>> + * just return 0.
>> + */
>> +static inline unsigned long random_get_entropy(void)
>> +{
>> +       if (unlikely(clint_time_val == NULL))
>> +               return 0;
>> +       return get_cycles();
>> +}
>> +
>>  #else /* CONFIG_RISCV_M_MODE */
>>
>>  static inline cycles_t get_cycles(void)
>>
>> >> Fixes: d5be89a8d118 ("RISC-V: Resurrect the MMIO timer implementation
>> >> for M-mode systems")
>> >> Signed-off-by: Anup Patel <anup.patel@wdc.com>
>> >> ---
>> >> Changes since v1:
>> >>  - Explicitly initialize clint_time_val to NULL in CLINT driver to
>> >>    avoid hang on Kendryte K210
>> >> ---
>> >>  arch/riscv/include/asm/timex.h    | 12 +++++++++---
>> >>  drivers/clocksource/timer-clint.c |  2 +-
>> >>  2 files changed, 10 insertions(+), 4 deletions(-)
>> >>
>> >> diff --git a/arch/riscv/include/asm/timex.h b/arch/riscv/include/asm/timex.h
>> >> index 7f659dda0032..6e7b04874755 100644
>> >> --- a/arch/riscv/include/asm/timex.h
>> >> +++ b/arch/riscv/include/asm/timex.h
>> >> @@ -17,18 +17,24 @@ typedef unsigned long cycles_t;
>> >>  #ifdef CONFIG_64BIT
>> >>  static inline cycles_t get_cycles(void)
>> >>  {
>> >> -    return readq_relaxed(clint_time_val);
>> >> +    if (clint_time_val)
>> >> +            return readq_relaxed(clint_time_val);
>> >> +    return 0;
>> >>  }
>> >>  #else /* !CONFIG_64BIT */
>> >>  static inline u32 get_cycles(void)
>> >>  {
>> >> -    return readl_relaxed(((u32 *)clint_time_val));
>> >> +    if (clint_time_val)
>> >> +            return readl_relaxed(((u32 *)clint_time_val));
>> >> +    return 0;
>> >>  }
>> >>  #define get_cycles get_cycles
>> >>
>> >>  static inline u32 get_cycles_hi(void)
>> >>  {
>> >> -    return readl_relaxed(((u32 *)clint_time_val) + 1);
>> >> +    if (clint_time_val)
>> >> +            return readl_relaxed(((u32 *)clint_time_val) + 1);
>> >> +    return 0;
>> >>  }
>> >>  #define get_cycles_hi get_cycles_hi
>> >>  #endif /* CONFIG_64BIT */
>> >> diff --git a/drivers/clocksource/timer-clint.c b/drivers/clocksource/timer-clint.c
>> >> index d17367dee02c..8dbec85979fd 100644
>> >> --- a/drivers/clocksource/timer-clint.c
>> >> +++ b/drivers/clocksource/timer-clint.c
>> >> @@ -37,7 +37,7 @@ static unsigned long clint_timer_freq;
>> >>  static unsigned int clint_timer_irq;
>> >>
>> >>  #ifdef CONFIG_RISCV_M_MODE
>> >> -u64 __iomem *clint_time_val;
>> >> +u64 __iomem *clint_time_val = NULL;
>>
>> This one I definately don't get.  According the internet, the C standard says
>>
>>     If an object that has static storage duration is not initialized
>>     explicitly, it is initialized implicitly as if every member that has
>>     arithmetic type were assigned 0 and every member that has pointer type were
>>     assigned a null pointer constant.
>>
>> so unless I'm missing something there shouldn't be any difference between these
>> two lines.  When I just apply this I get exactly the same "objdump -dt" before
>> and after.  I do see some difference in assembly, but only when I don't pass
>> "-fno-common" and that ends up being passed during my Linux builds.
>
> Even checkpatch complains about explicit NULL assignment to global variables.
>
> I will send v3 with a different approach to fix this so hopefully with
> v3 we will
> not require explicit NULL assignment.

Awesome, thanks!

>
> Regards,
> Anup
>
>>
>> >>  #endif
>> >>
>> >>  static void clint_send_ipi(const struct cpumask *target)
>> >
>> > For Kendryte:
>> >
>> > Tested-by: Damien Le Moal <damien.lemoal@wdc.com>
>> >
>> > --
>> > Damien Le Moal
>> > Western Digital