linux-kernel - Re: [v1 0/5] allow to reserve memory for normal kexec kernel

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Message-ID: <CA+CK2bBWis8TgyOmDhVgLYrOU95Za-UhSGSB3ufsjiNDt-Zd_w@mail.gmail.com>
Date:   Wed, 10 Jul 2019 11:56:50 -0400
From:   Pavel Tatashin <pasha.tatashin@...een.com>
To:     James Morse <james.morse@....com>
Cc:     Bhupesh Sharma <bhsharma@...hat.com>,
        James Morris <jmorris@...ei.org>,
        Sasha Levin <sashal@...nel.org>,
        Eric Biederman <ebiederm@...ssion.com>,
        kexec mailing list <kexec@...ts.infradead.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Jonathan Corbet <corbet@....net>,
        Catalin Marinas <catalin.marinas@....com>, will@...nel.org,
        Linux Doc Mailing List <linux-doc@...r.kernel.org>,
        linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>
Subject: Re: [v1 0/5] allow to reserve memory for normal kexec kernel

On Wed, Jul 10, 2019 at 11:19 AM James Morse <james.morse@....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.

Pasha