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

From: Samuel Holland <samuel.holland@sifive.com>
To: Deepak Gupta <debug@rivosinc.com>,
	Conor Dooley <conor@kernel.org>,
	Palmer Dabbelt <palmer@dabbelt.com>
Cc: linux-riscv@lists.infradead.org, devicetree@vger.kernel.org,
	Catalin Marinas <catalin.marinas@arm.com>,
	linux-kernel@vger.kernel.org, tech-j-ext@lists.risc-v.org,
	kasan-dev@googlegroups.com, Evgenii Stepanov <eugenis@google.com>,
	Krzysztof Kozlowski <krzysztof.kozlowski+dt@linaro.org>,
	Rob Herring <robh+dt@kernel.org>,
	Andrew Jones <ajones@ventanamicro.com>,
	Guo Ren <guoren@kernel.org>, Heiko Stuebner <heiko@sntech.de>,
	Paul Walmsley <paul.walmsley@sifive.com>
Subject: Re: [RISC-V] [tech-j-ext] [RFC PATCH 5/9] riscv: Split per-CPU and per-thread envcfg bits
Date: Thu, 21 Mar 2024 19:13:52 -0500	[thread overview]
Message-ID: <d9452ab4-a783-4bcf-ac25-40baa4f31fac@sifive.com> (raw)
In-Reply-To: <CAKC1njQYZHbQJ71mapeG1DEw=A+aGx77xsuQGecsNFpoJ=tzGQ@mail.gmail.com>

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

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