linux-kernel - Re: [PATCH RFC 01/14] x86/asm: add iosubmit_cmds512() based on movdir64b CPU instruction

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Message-ID: <7d3ba8bb-fbb6-097c-fa5a-6b3ec21f72e3@intel.com>
Date:   Wed, 20 Nov 2019 16:46:53 -0700
From:   Dave Jiang <dave.jiang@...el.com>
To:     "Hansen, Dave" <dave.hansen@...el.com>,
        "dmaengine@...r.kernel.org" <dmaengine@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "vkoul@...nel.org" <vkoul@...nel.org>
Cc:     "Williams, Dan J" <dan.j.williams@...el.com>,
        "Luck, Tony" <tony.luck@...el.com>,
        "Lin, Jing" <jing.lin@...el.com>,
        "Raj, Ashok" <ashok.raj@...el.com>,
        "Kumar, Sanjay K" <sanjay.k.kumar@...el.com>,
        "Dey, Megha" <megha.dey@...el.com>,
        "Pan, Jacob jun" <jacob.jun.pan@...el.com>,
        "Liu, Yi L" <yi.l.liu@...el.com>,
        "axboe@...nel.dk" <axboe@...nel.dk>,
        "akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        "mingo@...hat.com" <mingo@...hat.com>,
        "bp@...en8.de" <bp@...en8.de>,
        "Yu, Fenghua" <fenghua.yu@...el.com>,
        "hpa@...or.com" <hpa@...or.com>
Subject: Re: [PATCH RFC 01/14] x86/asm: add iosubmit_cmds512() based on
 movdir64b CPU instruction



On 11/20/19 2:50 PM, Hansen, Dave wrote:
> On 11/20/19 1:23 PM, Dave Jiang wrote:
>> +static inline void __iowrite512(void __iomem *__dst, const void *src)
>> +{
>> +	volatile struct { char _[64]; } *dst = __dst;
> 
> This _looks_ like gibberish.  I know it's not, but it is subtle enough
> that it really needs specific comments.

I'll add comments explaining.

> 
>> +static inline void iosubmit_cmds512(void __iomem *dst, const void *src,
>> +				    size_t count)
>> +{
>> +	const u8 *from = src;
>> +	const u8 *end = from + count * 64;
>> +
>> +	if (!cpu_has_write512())
>> +		return;
>> +
>> +	while (from < end) {
>> +		__iowrite512(dst, from);
>> +		from += 64;
>> +	}
>> +}
> 
> Won't this silently just drop things if the CPU doesn't have movdir64b
> support?
> 
> It seems like this shouldn't be called at all if
> !cpu_has_write512(), but wouldn't something like this be mroe appropriate?
> 
> 	if (!cpu_has_write512()) {
> 		WARN_ON_ONCE(1);
> 		return;
> 	}
> 
> Is the caller just supposed to infer that "dst" was never overwritten?
> 

Thanks. I'll add the WARN().