linux-kernel - Re: [PATCH v2 05/29] LoongArch: KVM: Add vcpu related header files

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Message-ID: <53900813-ff1b-2451-d962-6114b3d4ade0@redhat.com>
Date:   Tue, 21 Feb 2023 07:48:48 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     maobibo <maobibo@...ngson.cn>, Xi Ruoyao <xry111@...111.site>,
        Tianrui Zhao <zhaotianrui@...ngson.cn>
Cc:     Huacai Chen <chenhuacai@...nel.org>,
        WANG Xuerui <kernel@...0n.name>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        loongarch@...ts.linux.dev, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org, Jens Axboe <axboe@...nel.dk>,
        Mark Brown <broonie@...nel.org>,
        Alex Deucher <alexander.deucher@....com>,
        Oliver Upton <oliver.upton@...ux.dev>
Subject: Re: [PATCH v2 05/29] LoongArch: KVM: Add vcpu related header files

On 2/21/23 07:46, maobibo wrote:
>>> +       asm volatile (
>>> +               "parse_r __reg, %[val]\n\t"
>>> +               ".word 0x5 << 24 | %[reg] << 10 | 0 << 5 | __reg\n\t"
>> Don't do this.  You should add the instruction to binutils first, then
>> make CONFIG_KVM depend on the assembler supporting this instruction.
>> This is completely unreadable and only fine for an internal PoC.
> We are preparing to submit these instruction support for binutils,
> however it is still necessary. Supposing that it is supported in future
> gcc version, we can not drop existing gcc 12/13 supporting to compiling
> kernel with LoongArch architecture.
> 
> Maybe there will be human readable code like this:
> #if GCC_SUPPORT_KVM_INSTR
>    ...
> #else
>    asm volatile (".word   "
>    ...
> #endif

I agree, just add a comment with what would be the assembly code, i.e. 
something like

	/* Instructions only available in binutils v.... or later */
	asm volatile (
                "parse_r __reg, %[val]\n\t"
	       /* instrname %[val], %[reg] */
                ".word 0x5 << 24 | %[reg] << 10 | 0 << 5 | __reg\n\t"

Paolo