Re: [musl] riscv32 v2

[Palmer Dabbelt <palmerdabbelt@google.com>]

On Thu, 10 Sep 2020 03:01:31 PDT (-0700), vincenzo.frascino@arm.com wrote:
>
>
> On 9/10/20 8:36 AM, Arnd Bergmann wrote:
>> On Thu, Sep 10, 2020 at 1:08 AM Palmer Dabbelt <palmerdabbelt@google.com> wrote:
>>> On Wed, 09 Sep 2020 14:36:44 PDT (-0700), dalias@libc.org wrote:
>>>> On Wed, Sep 09, 2020 at 02:28:55PM -0700, Palmer Dabbelt wrote:
>>>>> On Wed, 09 Sep 2020 13:28:27 PDT (-0700), dalias@libc.org wrote:
>>>> Possible addition of vdso clock_gettime isn't a blocker for moving
>>>> forward with the musl port, but syscall_arch.h should accurately
>>>> describe what's available and should not attempt to use vdso before
>>>> it's a public kernel interface (e.g. resolving the question of what
>>>> the function name will be). So I think it should be removed for now.
>>>
>>> Sorry if that was confusing, but I definitely agree.
>>>
>>> I guess my point was that the lack of VDSO clock functions on rv32 was probably
>>> an oversight, but one that shouldn't block the port.  We definitely can't just
>>> make up a kernel interface, particularly as the reason we don't have these on
>>> rv32 is because the generic versions of the functions we're using don't appear
>>> to run on 32-bit targets.
>>>
>>> That probably means there's some more subtle issue, though TBH I don't know
>>> enough about the 64-bit-ification of time_t for it to just jump out at me.  I
>>> don't want to derail the thread too much, but I tried the obvious thing
>>
>> When the vdso for rv64 was added, there was no time64 support in the
>> vdso code in general, as this only came with the "generic vdso" infrastructure
>> that was added later on, with commit ad5d1122b82f ("riscv: use vDSO
>> common flow to reduce the latency of the time-related functions") in v5.8.
>>
>> At that point it probably should have been added as well.
>>
>>>     --- a/arch/riscv/kernel/vdso/Makefile
>>>     +++ b/arch/riscv/kernel/vdso/Makefile
>>>     @@ -7,9 +7,7 @@ ARCH_REL_TYPE_ABS := R_RISCV_32|R_RISCV_64|R_RISCV_JUMP_SLOT
>>>      include $(srctree)/lib/vdso/Makefile
>>>      # Symbols present in the vdso
>>>      vdso-syms  = rt_sigreturn
>>>     -ifdef CONFIG_64BIT
>>>      vdso-syms += vgettimeofday
>>>     -endif
>>>      vdso-syms += getcpu
>>>      vdso-syms += flush_icache
>>>
>>> and it doesn't build.  I've added Arnd, who might have a better idea of what's
>>> going on.  Whatever happens, I think the best bet is to just drop the clock
>>> functions (specifically __vdso_{clock_gettime,gettimeofday,clock_getres}) from
>>> the rv32 port right now.
>>
>> For rv32 you need clock_gettime64, not clock_gettime, which in the Linux
>> ABI refers to the interface with the old timespec. There was some debate
>> over whether clock_getres_time64 and gettimeofday_time64 would make
>> sense to be added here, but I have so far leaned to the position that these
>> are not as performance critical and not worth the effort.
>>
>> Vincenzo has argued that we might want to extend the generic vdso code
>> to include a number of additional syscall implementations, which would
>> then include gettimeofday_time64 and clock_getres_time64.
>>
>
> I agree with Arnd, clock_getres_time64 and gettimeofday_time64 were not added in
> the original port because not considered as performance critical as
> clock_gettime64. We might reconsider if there is a strong use case for those.

OK, seems reasonable to me.  I guess we can always add things later if they end
up being important, though I don't really have any feel for this sort of stuff
so I don't really have an opinion either way.

Thanks!

>
>>         Arnd
>>