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Subject: Re: [PATCH v2 16/16] arm64: Add ARM_SMCCC_ARCH_WORKAROUND_1 BP
 hardening support
To: Marc Zyngier <marc.zyngier@arm.com>,
        Ard Biesheuvel <ard.biesheuvel@linaro.org>
CC: Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
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        Mark Rutland <mark.rutland@arm.com>,
        "Robin Murphy" <robin.murphy@arm.com>, Jon Masters <jcm@redhat.com>
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From: Hanjun Guo <guohanjun@huawei.com>
Message-ID: <7e62d00e-df3b-1fb8-8e5e-83e1cf0e86bf@huawei.com>
Date: Fri, 2 Feb 2018 11:43:49 +0800
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On 2018/2/1 16:53, Marc Zyngier wrote:
[...]
>>>> ... and actually, perhaps it makes sense for the
>>>> SMCCC_ARCH_WORKAROUND_1 check to be completely independent of MIDR
>>>> based errata matching?
>>>>
>>>> I.e., if SMCCC v1.1 and SMCCC_ARCH_WORKAROUND_1 are both implemented,
>>>> we should probably invoke it even if the MIDR is not known to belong
>>>> to an affected implementation.
>>>
>>> This would have an impact on big-little systems, for which there is
>>> often a bunch of unaffected CPUs.
>>
>> I think it's what we are doing now, SMCCC v1.1 didn't provide the ability
>> to report per-cpu SMCCC_ARCH_WORKAROUND_1, and it said:
>>  - The discovery call must return the same result on all PEs in the system.
>>  - In heterogeneous systems with some PEs that require mitigation and others
>>    that do not, the firmware must provide a safe implementation of this
>>    function on all PEs.
>>
>> So from the spec that it's the firmware to take care of unaffected CPUs,
>> to the kernel it's the same.
> 
> The spec makes it safe. The MIDR list makes it fast.

Got it, thank you for clarifying this.

Thanks
Hanjun