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

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Date:	Wed, 16 Mar 2016 07:10:21 +0200
From:	Eli Cohen <eli@...lanox.com>
To:	Alexey Kardashevskiy <aik@...abs.ru>
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 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
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).

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