linux-kernel - Re: [PATCH v15 27/30] LoongArch: KVM: Implement vcpu world switch

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Date:   Wed, 28 Jun 2023 18:22:47 +0800
From:   WANG Xuerui <kernel@...0n.name>
To:     bibo mao <maobibo@...ngson.cn>,
        zhaotianrui <zhaotianrui@...ngson.cn>,
        Jinyang He <hejinyang@...ngson.cn>,
        linux-kernel@...r.kernel.org, kvm@...r.kernel.org
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Huacai Chen <chenhuacai@...nel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        loongarch@...ts.linux.dev, Jens Axboe <axboe@...nel.dk>,
        Mark Brown <broonie@...nel.org>,
        Alex Deucher <alexander.deucher@....com>,
        Oliver Upton <oliver.upton@...ux.dev>,
        Xi Ruoyao <xry111@...111.site>, tangyouling@...ngson.cn
Subject: Re: [PATCH v15 27/30] LoongArch: KVM: Implement vcpu world switch

On 2023/6/28 18:11, bibo mao wrote:
> 
> 
> 在 2023/6/28 17:51, WANG Xuerui 写道:
>> Hi,
>>
>> On 2023/6/28 16:34, zhaotianrui wrote:
>>>
>>> 在 2023/6/28 上午11:42, Jinyang He 写道:
>>>> On 2023-06-26 16:47, Tianrui Zhao wrote:
>>>>
>>>>> [snip]
>>>>
>>>>> +    ldx.d   t0, t1, t0
>>>>> +    csrwr    t0, LOONGARCH_CSR_PGDL
>>>>> +
>>>>> +    /* Mix GID and RID */
>>>>> +    csrrd        t1, LOONGARCH_CSR_GSTAT
>>>>> +    bstrpick.w    t1, t1, CSR_GSTAT_GID_SHIFT_END, CSR_GSTAT_GID_SHIFT
>>>>> +    csrrd        t0, LOONGARCH_CSR_GTLBC
>>>>> +    bstrins.w    t0, t1, CSR_GTLBC_TGID_SHIFT_END, CSR_GTLBC_TGID_SHIFT
>>>>> +    csrwr        t0, LOONGARCH_CSR_GTLBC
>>>>> +
>>>>> +    /*
>>>>> +     * Switch to guest:
>>>>> +     *  GSTAT.PGM = 1, ERRCTL.ISERR = 0, TLBRPRMD.ISTLBR = 0
>>>>> +     *  ertn
>>>>> +     */
>>>>> +
>>>>> +    /*
>>>>> +     * Enable intr in root mode with future ertn so that host interrupt
>>>>> +     * can be responsed during VM runs
>>>>> +     * guest crmd comes from separate gcsr_CRMD register
>>>>> +     */
>>>>> +    ori    t0, zero, CSR_PRMD_PIE
>>>> li.w t0, CSR_PRMD_PIE
>>> Thanks for your advice, and I think it need not to replace it with "li.w" there, as it has the same meaning with "ori" instruction, and "ori" instruction is simple and clear enough. The same as the following "move" instructions. What do you think of it.
>>
>> Just my 2c: I'd agree that pseudo-instructions should be used wherever possible and helping readability.
> "lu12i.w+srli.w" can be replaced by "li.w t0, KVM_GPGD"
> we accept the suggestion two instructions should be replaced with pseudo-instruction.
> 
> For the instruction "ori    t0, zero, CSR_PRMD_PIE"
> what is advantage of this pseudo-instruction
>      li.w t0, CSR_PRMD_PIE
> 
> is "ori t0, zero, CSR_PRMD_PIE" hard to understand? It is basic arithmetic instr and easy to understand also. To be frank I do not see the advantage of using li.w, also there is no document that pseudo-instruction should be used with high priority.

It depends on the reader. Sure the semantics are the same, but with "ori 
xx, zero, xx" someone's always going to wonder "why do 'x = 0 | 
something' when you can simply li.w", because even if it's easy to 
understand it's still an extra level of indirection.

And I've given the historical and general software engineering 
perspective too; it's not something set in stone, but I'd expect general 
software development best practices and minimizing any *possible* reader 
confusion to be acceptable.

-- 
WANG "xen0n" Xuerui

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