Re: [RISC-V] [tech-j-ext] [RFC PATCH 5/9] riscv: Split per-CPU and per-thread envcfg bits

[Samuel Holland]

On 2024-03-19 11:39 PM, Deepak Gupta wrote:
>>>> --- a/arch/riscv/include/asm/switch_to.h
>>>> +++ b/arch/riscv/include/asm/switch_to.h
>>>> @@ -69,6 +69,17 @@ static __always_inline bool has_fpu(void) { return false; }
>>>>  #define __switch_to_fpu(__prev, __next) do { } while (0)
>>>>  #endif
>>>>
>>>> +static inline void sync_envcfg(struct task_struct *task)
>>>> +{
>>>> +       csr_write(CSR_ENVCFG, this_cpu_read(riscv_cpu_envcfg) | task->thread.envcfg);
>>>> +}
>>>> +
>>>> +static inline void __switch_to_envcfg(struct task_struct *next)
>>>> +{
>>>> +       if (riscv_cpu_has_extension_unlikely(smp_processor_id(), RISCV_ISA_EXT_XLINUXENVCFG))
>>>
>>> I've seen `riscv_cpu_has_extension_unlikely` generating branchy code
>>> even if ALTERNATIVES was turned on.
>>> Can you check disasm on your end as well.  IMHO, `entry.S` is a better
>>> place to pick up *envcfg.
>>
>> The branchiness is sort of expected, since that function is implemented by
>> switching on/off a branch instruction, so the alternate code is necessarily a
>> separate basic block. It's a tradeoff so we don't have to write assembly code
>> for every bit of code that depends on an extension. However, the cost should be
>> somewhat lowered since the branch is unconditional and so entirely predictable.
>>
>> If the branch turns out to be problematic for performance, then we could use
>> ALTERNATIVE directly in sync_envcfg() to NOP out the CSR write.
> 
> Yeah I lean towards using alternatives directly.

One thing to note here: we can't use alternatives directly if the behavior needs
to be different on different harts (i.e. a subset of harts implement the envcfg
CSR). I think we need some policy about which ISA extensions are allowed to be
asymmetric across harts, or else we add too much complexity.

Regards,
Samuel