netdev - Re: [RFC PATCH 2/3] x86/io: implement 256-bit IO read and write

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Date:   Mon, 19 Mar 2018 15:43:10 +0100 (CET)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Rahul Lakkireddy <rahul.lakkireddy@...lsio.com>
cc:     x86@...nel.org, linux-kernel@...r.kernel.org,
        netdev@...r.kernel.org, mingo@...hat.com, hpa@...or.com,
        davem@...emloft.net, akpm@...ux-foundation.org,
        torvalds@...ux-foundation.org, ganeshgr@...lsio.com,
        nirranjan@...lsio.com, indranil@...lsio.com
Subject: Re: [RFC PATCH 2/3] x86/io: implement 256-bit IO read and write

On Mon, 19 Mar 2018, Rahul Lakkireddy wrote:

> Use VMOVDQU AVX CPU instruction when available to do 256-bit
> IO read and write.

That's not what the patch does. See below.

> Signed-off-by: Rahul Lakkireddy <rahul.lakkireddy@...lsio.com>
> Signed-off-by: Ganesh Goudar <ganeshgr@...lsio.com>

That Signed-off-by chain is wrong....

> +#ifdef CONFIG_AS_AVX
> +#include <asm/fpu/api.h>
> +
> +static inline u256 __readqq(const volatile void __iomem *addr)
> +{
> +	u256 ret;
> +
> +	kernel_fpu_begin();
> +	asm volatile("vmovdqu %0, %%ymm0" :
> +		     : "m" (*(volatile u256 __force *)addr));
> +	asm volatile("vmovdqu %%ymm0, %0" : "=m" (ret));
> +	kernel_fpu_end();
> +	return ret;

You _cannot_ assume that the instruction is available just because
CONFIG_AS_AVX is set. The availability is determined by the runtime
evaluated CPU feature flags, i.e. X86_FEATURE_AVX.

Aside of that I very much doubt that this is faster than 4 consecutive
64bit reads/writes as you have the full overhead of
kernel_fpu_begin()/end() for each access.

You did not provide any numbers for this so its even harder to
determine.

As far as I can tell the code where you are using this is a debug
facility. What's the point? Debug is hardly a performance critical problem.

Thanks,

	tglx