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From:   Ard Biesheuvel <ardb@kernel.org>
Date:   Tue, 19 Apr 2022 20:32:15 +0200
Message-ID: <CAMj1kXGyKQMeFWSK-s84pdL89qPTyTN_x3WHTgp_R7sH1+qOfA@mail.gmail.com>
Subject: Re: [PATCH v2 0/9] introduce mirrored memory support for arm64
To:     mawupeng <mawupeng1@huawei.com>
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        Catalin Marinas <catalin.marinas@arm.com>,
        Will Deacon <will@kernel.org>,
        Jonathan Corbet <corbet@lwn.net>,
        Thomas Gleixner <tglx@linutronix.de>,
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        Dave Hansen <dave.hansen@linux.intel.com>,
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        Darren Hart <dvhart@infradead.org>,
        Andy Shevchenko <andy@infradead.org>,
        Mike Rapoport <rppt@kernel.org>,
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On Sat, 16 Apr 2022 at 03:32, mawupeng <mawupeng1@huawei.com> wrote:
>
>
>
> =E5=9C=A8 2022/4/14 18:22, Ard Biesheuvel =E5=86=99=E9=81=93:
> > On Thu, 14 Apr 2022 at 11:54, Wupeng Ma <mawupeng1@huawei.com> wrote:
> >>
> >> From: Ma Wupeng <mawupeng1@huawei.com>
> >>
> >> Commit b05b9f5f9dcf ("x86, mirror: x86 enabling - find mirrored memory=
 ranges")
> >> introduced mirrored memory support for x86. This support rely on UEFI =
to
> >> report mirrored memory address ranges.  See UEFI 2.5 spec pages 157-15=
8:
> >>
> >>    http://www.uefi.org/sites/default/files/resources/UEFI%202_5.pdf
> >>
> >> Memory mirroring is a technique used to separate memory into two separ=
ate
> >> channels, usually on a memory device, like a server. In memory mirrori=
ng,
> >> one channel is copied to another to create redundancy. This method mak=
es
> >> input/output (I/O) registers and memory appear with more than one addr=
ess
> >> range because the same physical byte is accessible at more than one
> >> address. Using memory mirroring, higher memory reliability and a highe=
r
> >> level of memory consolidation are possible.
> >>
> >> Arm64 can support this too. So mirrored memory support is added to sup=
port
> >> arm64.
> >>
> >> Efi_fake_mem is used for testing mirrored features and will not be use=
d in
> >> production environment. This test features can fake memory's attribute
> >> values.
> >>
> >> The reason why efi_fake_mem support is put first is that memory's attr=
ibute
> >> is reported by BIOS which is hard to simulate. With this support, any =
arm64
> >> machines with efi support can easily test mirrored features.
> >>
> >> The main purpose of this patchset is to introduce mirrored support for
> >> arm64 and we have already fixed the problems we had which is shown in
> >> patch #5 to patch #7 and try to bring total isolation in patch #8 whic=
h
> >> will disable mirror feature if kernelcore is not specified.
> >>
> >> In order to test this support in arm64:
> >> - patch this patchset
> >> - add efi_fake_mem=3D8G@0:0x10000 in kernel parameter to simulate mirr=
ored
> >>    memroy between phy addr 0-8G.
> >> - add kernelcore=3Dmirror in kernel parameter
> >> - start you kernel
> >>
> >
> > As I explained before:
> >
> > - NAK to EFI fake_mem support on arm64
>
> fake_mem support on arm64 will be removed in subsequent version.
>
> > - NAK to the whole series until you come up with a proposal on how to
> > locate the static kernel image itself into more reliable memory, as
> > there is really no point to any of this otherwise.
>
> Sorry I am not familiar with this, as you metioned before,
>
>  > you have to iterate over the memory map and look for regions with
>  > the desired attribute, and allocate those pages explicitly.
>
> Do you mean this is x86, commit c05cd79750fb
> ("x86/boot/KASLR: Prefer mirrored memory regions for the kernel physical =
address").
> I will do some research.
>
>  > I'd prefer to implement this in the bootloader, and only add minimal
>  > logic to the stub to respect the placement of the kernel by the loader
>  > if the loader signals it to do so.
>
> Does this bootloader refer to grub and then add minimal logic to arm64-st=
ub.c?
>

Any bootloader, yes.

> What is the loader signal?

A protocol installed onto the image handle, as I suggested before. I
even cc'ed you on a patch that implements this.

> System exists mirrored memory reported by uefi?
>

What on earth is the point of any of this if the only use case being
targeted is efi_fake_mem with arbitrary fake mirrored regions?

So yes, unless there are systems that need this, I don't see a point
in merging any of this.