linux-kernel - Re: [PATCH v2 1/1] arm64: mm: correct the inside linear map boundaries during hotplug check

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Message-ID: <CA+CK2bDX9X+SdHOB=dd2kvsYfQwPvhuVNRByZW8sENuzuHRZyQ@mail.gmail.com>
Date:   Mon, 15 Feb 2021 17:28:18 -0500
From:   Pavel Tatashin <pasha.tatashin@...een.com>
To:     Ard Biesheuvel <ardb@...nel.org>
Cc:     Tyler Hicks <tyhicks@...ux.microsoft.com>,
        James Morris <jmorris@...ei.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Anshuman Khandual <anshuman.khandual@....com>,
        Mike Rapoport <rppt@...nel.org>,
        Logan Gunthorpe <logang@...tatee.com>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 1/1] arm64: mm: correct the inside linear map
 boundaries during hotplug check

> >
> > Btw, the KASLR check is incorrect: memstart_addr could also be
> > negative when running the 52-bit VA kernel on hardware that is only
> > 48-bit VA capable.
>
> Good point!
>
> if (IS_ENABLED(CONFIG_ARM64_VA_BITS_52) && (vabits_actual != 52))
>     memstart_addr -= _PAGE_OFFSET(48) - _PAGE_OFFSET(52);
>
> So, I will remove IS_ENABLED(CONFIG_RANDOMIZE_BASE) again.

Hi Ard,

Actually, looking more at this, I do not see how with 52VA on a 48VA
processor start offset can become negative unless randomization is
involved.
The start of the linear map will point to the first physical address
that is reported by memblock_start_of_DRAM(). However, memstart_addr
will be negative. So, I think the current approach using
IS_ENABLED(CONFIG_RANDOMIZE_BASE) is good.

48VA processor with VA_BITS_48:
memstart_addr 40000000
start_linear_pa 40000000
end_linear_pa 80003fffffff

48VA processor with VA_BITS_52:
memstart_addr fff1000040000000   <- Negative
start_linear_pa 40000000  <- positive, and the first PA address
end_linear_pa 80003fffffff

Thank you,
Pasha