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Subject: Re: [PATCH] arm64: cpufeature: check translation granule size based
 on kernel config
To: Leo Yan <leo.yan@linaro.org>
References: <1495102873-10858-1-git-send-email-leo.yan@linaro.org>
 <c0c8bd7b-647d-092d-65ac-08a90000e30c@arm.com>
 <20170518113627.GC25294@leoy-ThinkPad-T440>
Cc: Catalin Marinas <catalin.marinas@arm.com>,
        Will Deacon <will.deacon@arm.com>, Mark Rutland <mark.rutland@arm.com>,
        Marc Zyngier <marc.zyngier@arm.com>,
        Ard Biesheuvel <ard.biesheuvel@linaro.org>,
        James Morse <james.morse@arm.com>,
        linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org,
        Guodong Xu <guodong.xu@linaro.org>,
        Haojian Zhuang <haojian.zhuang@linaro.org>,
        Douglas Raillard <douglas.raillard@arm.com>
From: Suzuki K Poulose <Suzuki.Poulose@arm.com>
Message-ID: <9db90634-6bb1-23e0-5cc8-19c02eceac04@arm.com>
Date: Thu, 18 May 2017 13:39:12 +0100
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On 18/05/17 12:36, Leo Yan wrote:
> Hi Suzuki,
>
> On Thu, May 18, 2017 at 11:39:01AM +0100, Suzuki K Poulose wrote:
>> On 18/05/17 11:21, Leo Yan wrote:
>>> In the big.LITTLE system with two clusters, one is CA53 cluster and
>>> another is CA73 cluster. CA53 doesn't support 16KB memory translation
>>> granule size (4.3.21 AArch64 Memory Model Feature Register 0, EL1; ARM
>>> DDI 0500F), but CA73 supports this feature (4.3.27 AArch64 Memory Model
>>> Feature Register 0, EL1; ARM 100048_0002_04_en). As result, the kernel
>>> reports log for "Unexpected variation" as below.
>>>
>>> [    0.182113] CPU features: SANITY CHECK: Unexpected variation in SYS_ID_AA64MMFR0_EL1. Boot CPU: 0x00000000001122, CPU4: 0x00000000101122
>>
>>>
>>> This patch is to change the checking CPU feature for memory translation
>>> granule size based on kernel configuration. If kernel configuration has
>>> selected to use one specific memory translation granule size, then we
>>> will do strict sanity checking cross all CPUs. Otherwise we can skip to
>>> check unused features for memory translation granule size if kernel
>>> doesn't use it.
>>>
>>
>> If we were to suppress the warning (more on that below), we could simply
>> make this feature a NON_STRICT, since the unsupported CPUs won't boot
>> with 16K to hit this sanity check.
>>
>> However, there is a problem with disabling this warning. If a VM starts
>> using 16KB page size on a 4K/64K host, the VM could end up in unknown
>> failures when it switches to an unsupported CPU (after it has booted).
>> Of course the real fix lies in making the KVM exposing the safe value
>> for granule support to the VCPUs (which is currently being worked on by
>> Douglas in Cc). So, when we have that ready, we could make it NON_STRICT
>> instead of this approach.
>
> Thanks for the info :)
>
> I will use below patch for production branch temporarily. You could

Which production branch do you mean above ? Is it something in your intenral
repository ?

Suzuki