netdev - 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: <20160316060947.GD66645@x-vnc01.mtx.labs.mlnx>
Date:	Wed, 16 Mar 2016 08:09:47 +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 04:49:00PM +1100, Alexey Kardashevskiy wrote:
> 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?
User Access Region. It's the way you interface with the hardware.
> 
> >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...
>

It's not a performance issue. Defining 64KB for a UAR is not required
and wastes pci memory mapped i/o space.

> 
> >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.
>
It's not publically available. If you have an FAE that work with your
company you can ask him how to get the doc.
> 
> -- 
> Alexey
> --
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