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

From: "H. Peter Anvin" <[email-masked]>
Date: Tue, 21 Aug 2012 18:59:50 -0700

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