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

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Message-ID: <7d6125dd-29e8-14d8-b1d7-d8c14d7bec80@loongson.cn>
Date:   Tue, 21 Feb 2023 14:46:20 +0800
From:   maobibo <maobibo@...ngson.cn>
To:     Xi Ruoyao <xry111@...111.site>,
        Tianrui Zhao <zhaotianrui@...ngson.cn>,
        Paolo Bonzini <pbonzini@...hat.com>
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



在 2023/2/21 12:44, Xi Ruoyao 写道:
> On Mon, 2023-02-20 at 14:57 +0800, Tianrui Zhao wrote:
>> +/* GCSR */
>> +static inline u64 gcsr_read(u32 reg)
>> +{
>> +       u64 val = 0;
>> +
>> +       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

Regards
Bibo, Mao
> 
>> +               : [val] "+r" (val)
>> +               : [reg] "i" (reg)
>> +               : "memory");
>> +
>> +       return val;
>> +}
>