Date: Wed, 17 Apr 2024 10:41:29 +0200
From: Alexander Graf <graf@...zon.de>
To: Babis Chalios <bchalios@...zon.es>, <tytso@....edu>, <Jason@...c4.com>,
	<olivia@...enic.com>, <herbert@...dor.apana.org.au>, <robh@...nel.org>,
	<krzk+dt@...nel.org>, <conor+dt@...nel.org>, <linux-crypto@...r.kernel.org>,
	<devicetree@...r.kernel.org>, <linux-kernel@...r.kernel.org>
CC: <sudanl@...zon.com>, <xmarcalx@...zon.co.uk>, <dwmw@...zon.co.uk>
Subject: Re: [PATCH v5 4/5] dt-bindings: rng: Add vmgenid support


On 17.04.24 10:12, Babis Chalios wrote:
> From: Sudan Landge <sudanl@...zon.com>
>
> Virtual Machine Generation ID driver was introduced in commit af6b54e2b5ba
> ("virt: vmgenid: notify RNG of VM fork and supply generation ID"), as an
> ACPI only device.
>
> VMGenID specification http://go.microsoft.com/fwlink/?LinkId=260709 defines
> a mechanism for the BIOS/hypervisors to communicate to the virtual machine
> that it is executed with a different configuration (e.g. snapshot execution
> or creation from a template).
> The guest operating system can use the notification for various purposes
> such as re-initializing its random number generator etc.
>
> As per the specs, hypervisor should provide a globally unique identified,
> or GUID via ACPI.
>
> This patch tries to mimic the mechanism to provide the same functionality
> which is for a hypervisor/BIOS to notify the virtual machine when it is
> executed with a different configuration.
>
> As part of this support the devicetree bindings requires the hypervisors or
> BIOS to provide a memory address which holds the GUID and an IRQ which is
> used to notify when there is a change in the GUID.
> The memory exposed in the DT should follow the rules defined in the
> vmgenid spec mentioned above.
>
> *Reason for this change*:
> Chosing ACPI or devicetree is an intrinsic part of an hypervisor design.
> Without going into details of why a hypervisor would chose DT over ACPI,
> we would like to highlight that the hypervisors that have chose devicetree
> and now want to make use of the vmgenid functionality cannot do so today
> because vmgenid is an ACPI only device.
> This forces these hypervisors to change their design which could have
> undesirable impacts on their use-cases, test-scenarios etc.
>
> The point of vmgenid is to provide a mechanism to discover a GUID when
> the execution state of a virtual machine changes and the simplest
> way to do it is pass a memory location and an interrupt via devicetree.
> It would complicate things unnecessarily if instead of using devicetree,
> we try to implement a new protocol or modify other protocols to somehow
> provide the same functionility.
>
> We believe that adding a devicetree binding for vmgenid is a simpler,
> better alternative to provide the same functionality and will allow
> such hypervisors as mentioned above to continue using devicetree.
>
> More references to vmgenid specs:
>   - https://www.qemu.org/docs/master/specs/vmgenid.html
>   - https://learn.microsoft.com/en-us/windows/win32/hyperv_v2/virtual-
> machine-generation-identifier
>
> Signed-off-by: Sudan Landge <sudanl@...zon.com>


This was reviewed by Rob before. Make sure to propagate his Reviewed-by 
into new versions of the patch set unless you change the patch in 
question significantly enough that it invalidates the review.


Alex





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