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Message-ID: <0a83e174db16e15cb0f0d3ac37d6717c918ee78d.camel@infradead.org>
Subject: Re: [PATCH v1 0/4] virt: vmgenid: Add devicetree bindings support
From: David Woodhouse <dwmw2@infradead.org>
To: Rob Herring <robh@kernel.org>
Cc: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>, "Landge, Sudan"
	 <sudanl@amazon.co.uk>, Sudan Landge <sudanl@amazon.com>, tytso@mit.edu, 
	Jason@zx2c4.com, krzysztof.kozlowski+dt@linaro.org, conor+dt@kernel.org, 
	sathyanarayanan.kuppuswamy@linux.intel.com, thomas.lendacky@amd.com, 
	dan.j.williams@intel.com, devicetree@vger.kernel.org, 
	linux-kernel@vger.kernel.org, graf@amazon.de, bchalios@amazon.es, 
	xmarcalx@amazon.co.uk, ardb@kernel.org, benh <benh@kernel.crashing.org>
Date: Fri, 22 Mar 2024 14:27:22 +0000
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On Fri, 2024-03-22 at 08:22 -0500, Rob Herring wrote:
>=20
> > > What stops you from passing fw_cfg not to UEFI FW? BTW, no actual VM
> > > name was used in your posting, but now suddenly it is a talk about QE=
MU.

(Forgot to address the second part of that last time. No specific VMM
was mentioned in the first place because this isn't VMM-specific)

> > That would be possible. But not ideal.
>=20
> Why not ideal?
>=20
> To rephrase the question, why is it fine for UEFI to read the vmgenid
> from fw_cfg, but the kernel can't use the same mechanism?

Because fw_cfg an incestuous way to get data from the VMM into the BIOS
(both SeaBIOS and UEFI). It's the way we pass the ACPI tables and
things like that.

It *isn't* designed as a general-purpose way of doing device discovery
for use by various operating systems.

I'm also not sure Firecracker, which is the VMM Sudan is working on,
even *has* fw_cfg. Especially on ARM. If we're going to be forced to
add some complicated device with MMIO and DMA just to be able to
advertise the existence of a simple memory region, that's just as bad
as being forced to expose it as an emulated PCI device.

This is what DT is *for*.


> The response
> that you'd have to use UEFI to use fw_cfg makes no sense to me.=C2=A0The
> only reason I can think of is just being lazy and wanting to have
> minimal changes to some existing driver. It looks to me like you could
> implement this entirely in userspace already with zero kernel or
> binding changes. From a quick look, we already have a fw_cfg driver
> exposing UUID (that's the same thing as vmgenid AIUI) to userspace,
> and you can feed that back into the random pool.
>=20
> I am concerned that we already have a mechanism and you want to add a
> second way. When do we ever think that's a good idea? What happens=C2=A0
> on the next piece of fw_cfg data? We add yet another binding?

No, because fw_cfg is a way for the VMM to give configuration
information to the firmware. There's a clue in the name. The firmware
then sets up ACPI tables or DT to pass information in a more coherent
and structured fashion to general-purpose operating systems.

And some VMMs *don't* use fw_cfg at all because for the minimal microvm
case it's overkill.


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