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Message-ID: <60404403932a984d1f75d111ff53b9053af03579.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>, 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: Wed, 20 Mar 2024 16:55:45 +0000
In-Reply-To: <20240320161531.GA1810860-robh@kernel.org>
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	 <23692c07-98bd-477d-b244-bba14c50352c@linaro.org>
	 <38aad6c0e698c8e804694276d1762d61f2068ce8.camel@infradead.org>
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On Wed, 2024-03-20 at 11:15 -0500, Rob Herring wrote:
> On Wed, Mar 20, 2024 at 01:50:43PM +0000, David Woodhouse wrote:
> > On Tue, 2024-03-19 at 16:24 +0100, Krzysztof Kozlowski wrote:
> > > On 19/03/2024 15:32, Sudan Landge wrote:
> > > > This small series of patches aims to add devicetree bindings suppor=
t for
> > > > the Virtual Machine Generation ID (vmgenid) driver.
> > > >=20
> > > > Virtual Machine Generation ID driver was introduced in commit af6b5=
4e2b5ba
> > > > ("virt: vmgenid: notify RNG of VM fork and supply generation ID") a=
s an
> > > > ACPI only device.
> > > > We would like to extend vmgenid to support devicetree bindings beca=
use:
> > > > 1. A device should not be defined as an ACPI or DT only device.
>=20
> This (and the binding patch) tells me nothing about what "Virtual=20
> Machine Generation ID driver" is and isn't really justification for=20
> "why".

It's a reference to a memory area which the OS can use to tell whether
it's been snapshotted and restored (or 'forked'). A future submission
should have a reference to something like
https://www.qemu.org/docs/master/specs/vmgenid.html or the Microsoft
doc which is linked from there.

> DT/ACPI is for discovering what hardware folks failed to make=20
> discoverable. But here, both sides are software. Can't the software=20
> folks do better?

We are. Using device-tree *is* better. :)

> This is just the latest in $hypervisor bindings[1][2][3]. The value add=
=20
> must be hypervisors because every SoC vendor seems to be creating their=
=20
> own with their own interfaces.

The VMGenId one is cross-platform; we don't *want* to reinvent the
wheel there. We just want to discover that same memory area with
precisely the same semantics, but through the device-tree instead of
being forced to shoe-horn the whole of the ACPI horridness into a
platform which doesn't need it. (Or make it the BAR of a newly-invented
PCI device and have to add PCI to a microVM platform which doesn't
otherwise need it, etc.)

> I assume you have other calls into the hypervisor and notifications from=
=20
> the hypervisor? Are you going to add DT nodes for each one? I'd be more=
=20
> comfortable with DT describing THE communication channel with the=20
> hypervisor than what sounds like a singular function.

This isn't hypervisor-specific. There is a memory region with certain
semantics which may exist on all kinds of platforms, and we're just
allowing the guest to discover where it is. I don't see how it fits
into the model you're describing above.

> Otherwise, what's the next binding?

You meant that last as a rhetorical question, but I'll answer it
anyway. The thing I'm actually working on this week is a mechanism to
expose clock synchronisation (since it's kind of pointless for *all* of
the guests running on a host to run NTP/PTP/PPS/whatever to calibrate
the *same* underlying oscillator).

As far as the *discoverability* is concerned, it's fundamentally the
same thing =E2=80=94 just a memory region with certain defined semantics, a=
nd
probably an interrupt for when the contents change.

There *isn't* an ACPI specification for that one already; I was
thinking of describing it *only* in DT and if someone wants it on a
platform which is afflicted with ACPI, they can just do it in a PRP0001
device.

As with vmgenid, there's really very little benefit to wrapping a whole
bunch of pointless emulated "hardware discoverability" around it, when
it can just be described by ACPI/DT directly. That's what they're
*for*.


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