netdev - Re: [PATCH 0/3] x86_64, sfc: 128-bit memory-mapped I/O

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Message-ID: <50343D16.4040901@zytor.com>
Date:	Tue, 21 Aug 2012 18:59:50 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Ben Hutchings <bhutchings@...arflare.com>
CC:	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>, netdev@...r.kernel.org,
	linux-net-drivers@...arflare.com, x86@...nel.org
Subject: Re: [PATCH 0/3] x86_64, sfc: 128-bit memory-mapped I/O

On 08/21/2012 06:43 PM, Ben Hutchings wrote:
> On Tue, 2012-08-21 at 18:38 -0700, H. Peter Anvin wrote:
>> On 08/21/2012 06:17 PM, Ben Hutchings wrote:
>>> Current Solarflare network controllers have 128-bit memory-mapped
>>> registers which are normally accessed through a series of I/O
>>> operations.  However, it is also possible to access them with a single
>>> MOVAPS instruction on x86_64, and this is measurably faster as it
>>> requires only one PCIe transaction.
>>
>> Also, have you considered doing this with write combining instead?
> 
> We tried it, and it goes horribly wrong.  On some systems, the writes
> are not combined, but they are reordered in a way the hardware doesn't
> support.  See the comment at the top of drivers/net/ethernet/sfc/io.h.
> 

Yes, you have to make sure you properly enforce the necessary ordering
requirements manually (I think you can do that with sfence).

	-hpa



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