linux-kernel - Re: [PATCH v7 7/7] Documentation: KVM: Add hypercall for LoongArch

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Date: Sun, 24 Mar 2024 02:40:04 +0800
From: WANG Xuerui <kernel@...0n.name>
To: Bibo Mao <maobibo@...ngson.cn>, Huacai Chen <chenhuacai@...nel.org>,
 Tianrui Zhao <zhaotianrui@...ngson.cn>, Juergen Gross <jgross@...e.com>,
 Paolo Bonzini <pbonzini@...hat.com>, Jonathan Corbet <corbet@....net>
Cc: loongarch@...ts.linux.dev, linux-kernel@...r.kernel.org,
 virtualization@...ts.linux.dev, kvm@...r.kernel.org
Subject: Re: [PATCH v7 7/7] Documentation: KVM: Add hypercall for LoongArch

On 3/15/24 16:11, Bibo Mao wrote:
> [snip]
> +KVM hypercall ABI
> +=================
> +
> +Hypercall ABI on KVM is simple, only one scratch register a0 and at most
> +five generic registers used as input parameter. FP register and vector register
> +is not used for input register and should not be modified during hypercall.
> +Hypercall function can be inlined since there is only one scratch register.

Maybe it's better to describe the list of preserved registers with an 
expression such as "all non-GPR registers shall remain unmodified after 
returning from the hypercall", to guard ourselves against future ISA 
state additions.

But I still maintain that it's better to promise less here, and only 
hint on the extensive preservation of context as an implementation 
detail. It is for not losing our ability to save/restore less in the 
future, should we decide to do so.

-- 
WANG "xen0n" Xuerui

Linux/LoongArch mailing list: https://lore.kernel.org/loongarch/