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

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Message-ID: <20180219171852.jsbzawbsyv7v5yi7@armageddon.cambridge.arm.com>
Date:   Mon, 19 Feb 2018 17:18:52 +0000
From:   Catalin Marinas <catalin.marinas@....com>
To:     Shanker Donthineni <shankerd@...eaurora.org>
Cc:     Philip Elcan <pelcan@...eaurora.org>,
        Vikram Sethi <vikrams@...eaurora.org>,
        Marc Zyngier <marc.zyngier@....com>,
        Will Deacon <will.deacon@....com>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        kvmarm <kvmarm@...ts.cs.columbia.edu>,
        linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH] arm64: Add support for new control bits CTR_EL0.IDC and
 CTR_EL0.IDC

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).

-- 
Catalin