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From: Julien Grall <julien@xen.org>
To: Michal Orzel <michal.orzel@arm.com>, xen-devel@lists.xenproject.org
Cc: Andrew Cooper <andrew.cooper3@citrix.com>,
	George Dunlap <george.dunlap@citrix.com>,
	Jan Beulich <jbeulich@suse.com>,
	Stefano Stabellini <sstabellini@kernel.org>, Wei Liu <wl@xen.org>,
	Bertrand Marquis <bertrand.marquis@arm.com>,
	Volodymyr Babchuk <Volodymyr_Babchuk@epam.com>
Subject: Re: [PATCH] xen/arm: Allow setting the number of CPUs to activate at runtime
Date: Mon, 23 May 2022 21:00:39 +0100	[thread overview]
Message-ID: <3236e8e6-c25d-1733-9315-e0363cc73f84@xen.org> (raw)
In-Reply-To: <cb1e1ce0-4667-c436-6e5d-abc26add4ebe@arm.com>



On 23/05/2022 11:21, Michal Orzel wrote:
> Hi Julien,

Hi Michal,

> 
> On 23.05.2022 12:05, Julien Grall wrote:
>> Hi,
>>
>> On 23/05/2022 10:13, Michal Orzel wrote:
>>> Introduce a command line parameter "maxcpus" on Arm to allow adjusting
>>> the number of CPUs to activate.
>>
>> The current definition "maxcpus" is not really suitable for big.LITTLE systems as you have no flexibility to say how many types of each cores you want to boot.
>>
>> Instead, Xen will pick-up the first CPUs it parsed from the firmware tables.
>>
>>
>> So what's your use-case/target?
>>
> - use cases where we have no big little (although even on big.LITTLE limiting this number makes sense if we do not care about the types)

This may make sense in debug build, but for prod I think you need some 
certainty how which CPUs you are going to use.

So I would like a warning in the documentation "maxcpus" that in 
big.LITTLE system, there are no guarantee on which types will be used.

This is technically a lie, but I don't want a user to start relying on 
how Xen will parse the DT.

> - debug cases where we want to set maxcpus=1

Thanks for the clarification. I would be happy to add my tag with a 
warning in the documentation.

Cheers,

-- 
Julien Grall


  reply	other threads:[~2022-05-23 20:01 UTC|newest]

Thread overview: 9+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2022-05-23  9:13 [PATCH] xen/arm: Allow setting the number of CPUs to activate at runtime Michal Orzel
2022-05-23 10:05 ` Julien Grall
2022-05-23 10:21   ` Michal Orzel
2022-05-23 20:00     ` Julien Grall [this message]
2022-05-24  6:34       ` Michal Orzel
2022-05-26  9:18         ` Michal Orzel
2022-05-27 17:53         ` Julien Grall
2022-05-30  9:09 ` Bertrand Marquis
2022-06-08 10:17   ` Julien Grall

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