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Subject: Re: [v1 0/5] allow to reserve memory for normal kexec kernel
To:     Pavel Tatashin <pasha.tatashin@soleen.com>,
        James Morse <james.morse@arm.com>
Cc:     Sasha Levin <sashal@kernel.org>, Jonathan Corbet <corbet@lwn.net>,
        Catalin Marinas <catalin.marinas@arm.com>,
        Bhupesh Sharma <bhsharma@redhat.com>,
        Linux Doc Mailing List <linux-doc@vger.kernel.org>,
        kexec mailing list <kexec@lists.infradead.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        James Morris <jmorris@namei.org>,
        Eric Biederman <ebiederm@xmission.com>, will@kernel.org,
        linux-arm-kernel <linux-arm-kernel@lists.infradead.org>
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From:   Vladimir Murzin <vladimir.murzin@arm.com>
Message-ID: <93f99d54-9fe4-a191-4877-080ad78322bb@arm.com>
Date:   Thu, 11 Jul 2019 09:12:05 +0100
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Hi,

On 7/10/19 4:56 PM, Pavel Tatashin wrote:
> On Wed, Jul 10, 2019 at 11:19 AM James Morse <james.morse@arm.com> wrote:
>>
>> Hi Pasha,
>>
>> On 09/07/2019 14:07, Pavel Tatashin wrote:
>>>>> Enabling MMU and D-Cache for relocation  would essentially require the
>>>>> same changes in kernel. Could you please share exactly why these were
>>>>> not accepted upstream into kexec-tools?
>>>>
>>>> Because '--no-checks' is a much simpler alternative.
>>>>
>>>> More of the discussion:
>>>> https://lore.kernel.org/linux-arm-kernel/5599813d-f83c-d154-287a-c131c48292ca@arm.com/
>>>>
>>>> While you can make purgatory a fully-fledged operating system, it doesn't really need to
>>>> do anything on arm64. Errata-workarounds alone are a reason not do start down this path.
>>>
>>> Thank you James. I will summaries the information gathered from the
>>> yesterday's/today's discussion and add it to the cover letter together
>>> with ARM64 tag. I think, the patch series makes sense for ARM64 only,
>>> unless there are other platforms that disable caching/MMU during
>>> relocation.
>>
>> I'd prefer not to reserve additional memory for regular kexec just to avoid the relocation.
>> If the kernel's relocation work is so painful we can investigate doing it while the MMU is
>> enabled. If you can compare regular-kexec with kexec_file_load() you eliminate the
>> purgatory part of the work.
> 
> Relocation time is exactly the same for regular-kexec and
> kexec_file_load(). So, the relocation is indeed painful for our case.
> I am working on adding MMU enabled kernel relocation.

Out of curiosity, does enabling only I-cache make a difference? IIRC, it doesn't
require setting MMU, in contrast to D-cache.

Cheers
Vladimir

> 
> Pasha
> 
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