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Subject: Re: [PATCH 15/15] riscv: disable the EFI PECOFF header for M-mode
From:   Troy Benjegerdes <troy.benjegerdes@sifive.com>
In-Reply-To: <4f1677e24a5fcdfd2fda714cdd66f4dbe7817284.camel@wdc.com>
Date:   Wed, 21 Aug 2019 10:54:44 -0700
Cc:     "hch@lst.de" <hch@lst.de>,
        "paul.walmsley@sifive.com" <paul.walmsley@sifive.com>,
        "linux-riscv@lists.infradead.org" <linux-riscv@lists.infradead.org>,
        Damien Le Moal <Damien.LeMoal@wdc.com>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "palmer@sifive.com" <palmer@sifive.com>
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References: <20190813154747.24256-1-hch@lst.de>
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To:     Atish Patra <Atish.Patra@wdc.com>
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> On Aug 21, 2019, at 10:31 AM, Atish Patra <Atish.Patra@wdc.com> wrote:
>=20
> On Tue, 2019-08-20 at 21:14 -0700, Troy Benjegerdes wrote:
>>> On Aug 13, 2019, at 8:47 AM, Christoph Hellwig <hch@lst.de> wrote:
>>>=20
>>> No point in bloating the kernel image with a bootloader header if
>>> we run bare metal.
>>=20
>> I would say the same for S-mode. EFI booting should be an option, not
>> a requirement.=20
>=20
> EFI booting is never a requirement on any board. When EFI stub will be
> added for kernel, it will be enabled with CONFIG_EFI_STUB only.=20
>=20
> The current additional header is only 64 bytes and also required for
> booti in U-boot. So it shouldn't disabled for S-mode.
>=20
> Disabling it for M-Mode Linux is okay because of memory constraint and
> M-Mode linux won't use U-boot anyways.
>=20
>> I have M-mode U-boot working with bootelf to start BBL,
>> and at some point, I=E2=80=99m hoping we can have a M-mode linux =
kernel be
>> the SBI provider for S-mode kernels,=20
>=20
> Why do you want bloat a M-Mode software with Linux just for SBI
> implementation?
>=20
> Using Linux as a last stage boot loader i.e. LinuxBoot may make sense
> though.
>=20

Boot time, and ease of development, and simplified system management.

Having M-mode linux as a supervisor/boot kernel can get us to responding
to HTTPS/SSH/etc requests within seconds of power-on, while the =
=E2=80=98boot=E2=80=99
kernel can be loading guest S-mode kernels from things like NVME flash
drives that are going to be a lot more code and development to support =
in
U-boot or any other non-linux dedicated boot loader.

There=E2=80=99s also a very strong security argument, as Linux is going =
to get the
largest and broadest security review, and will likely get software =
updates
a lot faster than dedicated boot firmwares will.

Another reason would be sharing the same kernel binary (elf file) for =
both
M-mode, and S-mode, and using the device tree passed to each to specify
which mode it should be running it. There are probably a bunch of =
gotchas
with this idea, and even so I suspect someone will decide to go ahead =
and
just do it eventually because it could make testing, validation, and =
security
updates a lot easier from an operational/deployment point of view.

Linuxbios convinced me that if you want to do a really large cluster,
you can build, manage, and run such a thing with fewer people and
engineering cost than if you have all these extra layers of boot =
firmware
that require some company to have firmware engineers and lots of extra
system testing on the firmware.=