netdev - Re: [net-next-2.6 PATCH v2] x86: Align skb w/ start of cacheline on newer core 2/Xeon Arch

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Message-ID: <4C2CFC87.6020003@intel.com>
Date:	Thu, 01 Jul 2010 13:37:27 -0700
From:	Alexander Duyck <alexander.h.duyck@...el.com>
To:	Andi Kleen <andi@...stfloor.org>
CC:	"Kirsher, Jeffrey T" <jeffrey.t.kirsher@...el.com>,
	"davem@...emloft.net" <davem@...emloft.net>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	"gospo@...hat.com" <gospo@...hat.com>,
	"bphilips@...ell.com" <bphilips@...ell.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>, "x86@...nel.org" <x86@...nel.org>
Subject: Re: [net-next-2.6 PATCH v2] x86: Align skb w/ start of cacheline
 on newer core 2/Xeon Arch

Andi Kleen wrote:
> Jeff Kirsher <jeffrey.t.kirsher@...el.com> writes:
> 
> 
> Sorry for the late comment.
> 
>>  
>> +#ifdef CONFIG_MCORE2
>> +/*
>> + * We handle most unaligned accesses in hardware.  On the other hand
>> + * unaligned DMA can be quite expensive on some Nehalem processors.
>> + *
>> + * Based on this we disable the IP header alignment in network drivers.
>> + */
>> +#define NET_IP_ALIGN	0
>> +#endif
>>  #endif /* _ASM_X86_SYSTEM_H */
> 
> The ifdef should be imho dropped and the option be made unconditional
> for all x86. I am not aware of any x86 core where unalignment is really
> slow. This would increase the chance of it actually working on many
> configurations which do not necessarily optimize for Core2.
> 
> -Andi

Seems to make sense to me.  I will see about generating a patch that 
drops the check for CONFIG_MCORE2.

Thanks,

Alex
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