linux-kernel - Re: [RFC PATCH kernel] Revert "net/mlx4_core: Set UAR page size to 4KB regardless of system page size"

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Message-ID: <56E8F3CC.3070203@ozlabs.ru>
Date:	Wed, 16 Mar 2016 16:49:00 +1100
From:	Alexey Kardashevskiy <aik@...abs.ru>
To:	Eli Cohen <eli@...lanox.com>
Cc:	Or Gerlitz <gerlitz.or@...il.com>, Huy Nguyen <huyn@...lanox.com>,
	Doug Ledford <dledford@...hat.com>,
	Eugenia Emantayev <eugenia@...lanox.com>,
	Hal Rosenstock <hal.rosenstock@...il.com>,
	Sean Hefty <sean.hefty@...el.com>,
	Yishai Hadas <yishaih@...lanox.com>,
	Linux Kernel <linux-kernel@...r.kernel.org>,
	"linux-rdma@...r.kernel.org" <linux-rdma@...r.kernel.org>,
	Linux Netdev List <netdev@...r.kernel.org>,
	Paul Mackerras <paulus@...ba.org>,
	Carol L Soto <clsoto@...ibm.com>,
	Yevgeny Petrilin <yevgenyp@...lanox.com>,
	Brian J King <bjking1@...ibm.com>
Subject: Re: [RFC PATCH kernel] Revert "net/mlx4_core: Set UAR page size to
 4KB regardless of system page size"

On 03/16/2016 04:10 PM, Eli Cohen wrote:
> On Wed, Mar 16, 2016 at 01:07:58PM +1100, Alexey Kardashevskiy wrote:
>>
>> So with v4.5 as a host, there is no actual distro available today to
>> use as a guest in the next 6 months (or whatever it takes to
>> backport this partucular patch back there).
>>
>> You could have added a module parameter to enforce the old behavoir,
>> at least...
>>
>> And sorry but from the original commit log I could not understand
>> why exactly all existing guests need to be broken. Could you please
>> point me to a piece of documentation describing all this UAR
>> bisuness (what is UAR, why 128 UARs are required and for what, etc).
>> Thanks.
>>
>
> We are going to send a patch that fixes this using a module parameter.
> The patch will be on top of Huy's patch.
>
> Some background to the problem: mlx4 supported devices require 128 UAR

What does UAR stand for?

> pages from PCI memory space defined by BAR2-3. Each UAR page can be
> any power of 2 value from 4K up to 64K. Before Huy's patch the driver
> chose UAR page size to be equal to system page size. Since PowerPC's
> page size is 64K this means minimum requirement of UAR pages is not
> met (default UAR BAR is 8MB and only half of it is really reserved for
> UARs).

And what was the downside? afaict the performance was good...


> More details can be found in the programmer's manual.

Can you please point me to this manual on the website? I tried, honestly, 
could not find it. Thanks.


-- 
Alexey