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

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Date:   Wed, 28 Jun 2023 18:40:20 +0800
From:   Xi Ruoyao <xry111@...111.site>
To:     WANG Xuerui <kernel@...0n.name>, 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>, tangyouling@...ngson.cn
Subject: Re: [PATCH v15 27/30] LoongArch: KVM: Implement vcpu world switch

On Wed, 2023-06-28 at 18:22 +0800, WANG Xuerui wrote:
> > 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.

The pseudo *should* be the high priority in general.  If we don't
consider the pseudos high priority, people will start to load immediates
with "their own way", like writing "addi.w $t0, $zero, 42" or even "xori
$t0, $zero, 42" for "42".  These may puzzle the uarch, causing a
performance worse than "ori $t0, $zero, 42".  So the only rationale
things are either:

1. Telling people always use pseudos if possible.

or

2. Telling people "remember to use ori for loading small immediates, not
other instructions".

1 is obviously easier.

-- 
Xi Ruoyao <xry111@...111.site>
School of Aerospace Science and Technology, Xidian University