linux-kernel - Re: [PATCH -next] RISC-V: KVM: optimize kvm_arch_hardware

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Date:   Mon, 21 Nov 2022 21:38:12 +0800
From:   chenlifu <chenlifu@...wei.com>
To:     Andrew Jones <ajones@...tanamicro.com>
CC:     <anup@...infault.org>, <atishp@...shpatra.org>,
        <paul.walmsley@...ive.com>, <palmer@...belt.com>,
        <aou@...s.berkeley.edu>, <kvm@...r.kernel.org>,
        <kvm-riscv@...ts.infradead.org>, <linux-riscv@...ts.infradead.org>,
        <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH -next] RISC-V: KVM: optimize kvm_arch_hardware_enable()

在 2022/11/21 18:53, Andrew Jones 写道:
> On Mon, Nov 21, 2022 at 08:39:15AM +0800, Chen Lifu wrote:
>> The values of CSR_HEDELEG and CSR_HIDELEG registers are constants,
>> so change them from variables to macros.
>>
>> Signed-off-by: Chen Lifu <chenlifu@...wei.com>
>> ---
>>   arch/riscv/kvm/main.c | 31 +++++++++++++------------------
>>   1 file changed, 13 insertions(+), 18 deletions(-)
>>
>> diff --git a/arch/riscv/kvm/main.c b/arch/riscv/kvm/main.c
>> index df2d8716851f..70196b03b6f9 100644
>> --- a/arch/riscv/kvm/main.c
>> +++ b/arch/riscv/kvm/main.c
>> @@ -12,10 +12,21 @@
>>   #include <linux/kvm_host.h>
>>   #include <asm/csr.h>
>>   #include <asm/hwcap.h>
>>   #include <asm/sbi.h>
>>   
>> +#define EXC_HEDELEG ((1UL << EXC_INST_MISALIGNED) | \
>> +		     (1UL << EXC_BREAKPOINT)      | \
>> +		     (1UL << EXC_SYSCALL)         | \
>> +		     (1UL << EXC_INST_PAGE_FAULT) | \
>> +		     (1UL << EXC_LOAD_PAGE_FAULT) | \
>> +		     (1UL << EXC_STORE_PAGE_FAULT))
>> +
>> +#define IRQ_HIDELEG ((1UL << IRQ_VS_SOFT)  | \
>> +		     (1UL << IRQ_VS_TIMER) | \
>> +		     (1UL << IRQ_VS_EXT))
>> +
>>   long kvm_arch_dev_ioctl(struct file *filp,
>>   			unsigned int ioctl, unsigned long arg)
>>   {
>>   	return -EINVAL;
>>   }
>> @@ -30,29 +41,13 @@ int kvm_arch_hardware_setup(void *opaque)
>>   	return 0;
>>   }
>>   
>>   int kvm_arch_hardware_enable(void)
>>   {
>> -	unsigned long hideleg, hedeleg;
>> -
>> -	hedeleg = 0;
>> -	hedeleg |= (1UL << EXC_INST_MISALIGNED);
>> -	hedeleg |= (1UL << EXC_BREAKPOINT);
>> -	hedeleg |= (1UL << EXC_SYSCALL);
>> -	hedeleg |= (1UL << EXC_INST_PAGE_FAULT);
>> -	hedeleg |= (1UL << EXC_LOAD_PAGE_FAULT);
>> -	hedeleg |= (1UL << EXC_STORE_PAGE_FAULT);
>> -	csr_write(CSR_HEDELEG, hedeleg);
>> -
>> -	hideleg = 0;
>> -	hideleg |= (1UL << IRQ_VS_SOFT);
>> -	hideleg |= (1UL << IRQ_VS_TIMER);
>> -	hideleg |= (1UL << IRQ_VS_EXT);
>> -	csr_write(CSR_HIDELEG, hideleg);
>> -
>> +	csr_write(CSR_HEDELEG, EXC_HEDELEG);
>> +	csr_write(CSR_HIDELEG, IRQ_HIDELEG);
>>   	csr_write(CSR_HCOUNTEREN, -1UL);
>> -
>>   	csr_write(CSR_HVIP, 0);
>>   
>>   	return 0;
>>   }
>>   
>> -- 
>> 2.37.1
>>
> 
> I don't think this optimizes anything. I'm pretty sure the compiler will
> load the input to csr_write() in the most efficient way it can, regardless
> of using a constant or building the input out of constants. This could
> maybe be considered a code cleanup, but, in this case, it's really in the
> eye of the beholder.
> 
> Thanks,
> drew

Thanks drew，you're right.
I compared two version arch/riscv/kvm/main.o files, they are same.