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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Date:	Wed, 16 Mar 2016 13:07:58 +1100
From:	Alexey Kardashevskiy <aik@...abs.ru>
To:	Or Gerlitz <gerlitz.or@...il.com>, Huy Nguyen <huyn@...lanox.com>
Cc:	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 03/15/2016 09:40 PM, Or Gerlitz wrote:
> On Tue, Mar 15, 2016 at 12:19 PM, Alexey Kardashevskiy <aik@...abs.ru> wrote:
>> This reverts commit 85743f1eb34548ba4b056d2f184a3d107a3b8917.
>>
>> Without this revert, POWER "pseries" KVM guests with a VF passed to a guest
>> using VFIO fail to bring the driver up:
>>
>> mlx4_core: Mellanox ConnectX core driver v2.2-1 (Feb, 2014)
>> mlx4_core: Initializing 0000:00:00.0
>> mlx4_core 0000:00:00.0: enabling device (0000 -> 0002)
>> mlx4_core 0000:00:00.0: Detected virtual function - running in slave mode
>> mlx4_core 0000:00:00.0: Sending reset
>> mlx4_core 0000:00:00.0: Sending vhcr0
>> mlx4_core 0000:00:00.0: HCA minimum page size:512
>> mlx4_core 0000:00:00.0: UAR size:4096 != kernel PAGE_SIZE of 65536
>> mlx4_core 0000:00:00.0: Failed to obtain slave caps
>
>> Both host and guest use 64K system pages.
>>
>> How to fix this properly? Thanks.
>
> The commit message says:
>
> "[..] Regarding backward compatibility in SR-IOV, if hypervisor has
> this new code, the virtual OS must be updated. [...]"


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.


-- 
Alexey