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

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Date:   Wed, 28 Jun 2023 17:51:28 +0800
From:   WANG Xuerui <kernel@...0n.name>
To:     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>, maobibo@...ngson.cn,
        Xi Ruoyao <xry111@...111.site>, tangyouling@...ngson.cn
Subject: Re: [PATCH v15 27/30] LoongArch: KVM: Implement vcpu world switch

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.

FYI there were similar usages way before, but all were cleaned up with 
my previous commit 57ce5d3eefac ("LoongArch: Use the "move" 
pseudo-instruction where applicable").

Such usages apparently came from an era when the LoongArch toolchains 
didn't support any pseudo-instruction, and are less intuitive especially 
for someone not familiar with LoongArch assembly. Given that familiarity 
with LoongArch won't be widespread (unlike with e.g. RISC-V that are 
adopted more widely), we should optimize for readability when writing 
code; it's also a best practice in general because code is read way more 
often than written, and people care about semantics not unnecessary 
details like "how are moves like this or that materialized".

-- 
WANG "xen0n" Xuerui

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