linux-kernel - Re: [PATCH v7 00/25] arm64: MMU enabled kexec relocation

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Message-ID: <CA+CK2bC2KwWufE1DWa4szn_hQ1dbjDVHgYUu7=J4O_kvKXTrHg@mail.gmail.com>
Date:   Thu, 17 Oct 2019 10:26:33 -0400
From:   Pavel Tatashin <pasha.tatashin@...een.com>
To:     Pavel Tatashin <pasha.tatashin@...een.com>,
        James Morris <jmorris@...ei.org>,
        Sasha Levin <sashal@...nel.org>,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        kexec mailing list <kexec@...ts.infradead.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Jonathan Corbet <corbet@....net>,
        Catalin Marinas <catalin.marinas@....com>, will@...nel.org,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        Marc Zyngier <marc.zyngier@....com>,
        James Morse <james.morse@....com>,
        Vladimir Murzin <vladimir.murzin@....com>,
        Matthias Brugger <matthias.bgg@...il.com>,
        Bhupesh Sharma <bhsharma@...hat.com>,
        linux-mm <linux-mm@...ck.org>,
        Mark Rutland <mark.rutland@....com>, steve.capper@....com,
        rfontana@...hat.com, Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH v7 00/25] arm64: MMU enabled kexec relocation

> https://lore.kernel.org/lkml/45a2f0b8-5bac-8b5d-d595-f92e9acb27ad@arm.com
> > +     /* Map relocation function va == pa */
> > +     rc = trans_pgd_map_page(&info, trans_ttbr0,  __va(kern_reloc),
> > +                             kern_reloc, PAGE_KERNEL_EXEC);
> > +     if (rc)
> > +             return rc;
> James wrote:
> You can't do this with the page table helpers. We support platforms
> with no memory in range of TTBR0's VA space. See dd006da21646f
>
> You will need some idmapped memory to turn the MMU off on a system
> that booted at EL1. This will need to be in a set of page tables
> that the helpers can't easily touch - so it should only be a single
> page. (like the arch code's existing idmap - although that may
> have been overwritten).
>
> (I have a machine where this is a problem, if I get the time I will
> have a stab at making hibernate's safe page idmaped).
> ---
>
> As I understand, there are platforms where TTBR0 cannot include all
> physical memory for idmap. However, kexec must have at least one page
> idmapped (kimage->control_code_page) to be able to relocate kernel
> while MMU is enabled:
>
> I am still trying to fully understand the problem:
> CONFIG_ARM64_VA_BITS must be smaller than 48 and physical memory must
> start at a high address for this problem to occur.
>
> Why can't we simply decrease T0SZ to cover all physical memory?

Is there a way to reproduce this platform with qemu?

Pasha