Re: [PATCH] arm64: Add support for new control bits CTR_EL0.IDC and CTR_EL0.IDC

From: Shanker Donthineni <shankerd@codeaurora.org>
To: Catalin Marinas <catalin.marinas@arm.com>
Cc: Philip Elcan <pelcan@codeaurora.org>,
	Vikram Sethi <vikrams@codeaurora.org>,
	Marc Zyngier <marc.zyngier@arm.com>,
	Will Deacon <will.deacon@arm.com>,
	linux-kernel <linux-kernel@vger.kernel.org>,
	kvmarm <kvmarm@lists.cs.columbia.edu>,
	linux-arm-kernel <linux-arm-kernel@lists.infradead.org>
Subject: Re: [PATCH] arm64: Add support for new control bits CTR_EL0.IDC and CTR_EL0.IDC
Date: Mon, 19 Feb 2018 12:30:46 -0600	[thread overview]
Message-ID: <da2b54da-cb05-b478-ce61-eeb3308de672@codeaurora.org> (raw)
In-Reply-To: <20180219171852.jsbzawbsyv7v5yi7@armageddon.cambridge.arm.com>

Thanks Catalin for your comments.

On 02/19/2018 11:18 AM, Catalin Marinas wrote:
> On Mon, Feb 19, 2018 at 10:35:30AM -0600, Shanker Donthineni wrote:
>> On 02/19/2018 08:38 AM, Catalin Marinas wrote:
>>> On the patch, I'd rather have an alternative framework entry for no VAU
>>> cache maint required and some ret instruction at the beginning of the
>>> cache maint function rather than jumping out of the loop somewhere
>>> inside the cache maintenance code, penalising the CPUs that do require
>>> it.
>>
>> Alternative framework might break things in case of CPU hotplug. I need one
>> more confirmation from you on incorporating alternative framework.     
> 
> CPU hotplug can be an issue but it should be handled like other similar
> cases: if a CPU comes online late and its features are incompatible, it
> should not be brought online. The cpufeature code handles this.
> 
> With Will's patch for CTR_EL0, we handle different CPU features during
> boot, defaulting to the lowest value for the IDC/DIC bits.
> 
> I suggest you add new ARM64_HAS_* feature bits and enable them based on
> CTR_EL0.IDC and DIC. You could check for both being 1 with a single
> feature bit but I guess an implementation is allowed to have these
> different (e.g. DIC == 0 and IDC == 1).
> 

I'll add two new features ARM64_HAS_DIC and ARM64_HAS_IDC to support
all implementations. Unfortunately QCOM server chips supports IDC not DIC.

-- 
Shanker Donthineni
Qualcomm Datacenter Technologies, Inc. as an affiliate of Qualcomm Technologies, Inc.
Qualcomm Technologies, Inc. is a member of the Code Aurora Forum, a Linux Foundation Collaborative Project.