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Subject: Re: [PATCH v7 2/3] dt-bindings: rng: Add vmgenid support
Message-ID: <bf9b59ed-a739-4a06-93db-facb0c89dab6@amazon.co.uk>
Date: Fri, 19 Apr 2024 14:31:45 +0100
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To: "Jason A. Donenfeld" <Jason@zx2c4.com>, <tytso@mit.edu>,
	<robh@kernel.org>, <krzk@kernel.org>, <conor+dt@kernel.org>,
	<devicetree@vger.kernel.org>, <linux-kernel@vger.kernel.org>,
	<sudanl@amazon.com>, <graf@amazon.com>, <dwmw@amazon.co.uk>,
	<krzysztof.kozlowski@linaro.org>, <bchalios@amazon.es>,
	<xmarcalx@amazon.co.uk>
References: <20240418121249.42380-1-Jason@zx2c4.com>
 <20240418121249.42380-3-Jason@zx2c4.com>
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From: "Landge, Sudan" <sudanl@amazon.co.uk>
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On 18/04/2024 13:12, Jason A. Donenfeld wrote:
> CAUTION: This email originated from outside of the organization. Do not click links or open attachments unless you can confirm the sender and know the content is safe.
> 
> 
> 
> From: Sudan Landge <sudanl@amazon.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 the vmgenid specs are found below.
> 
> Signed-off-by: Sudan Landge <sudanl@amazon.com>
> Reviewed-by: Rob Herring <robh@kernel.org>
> Reviewed-by: Alexander Graf <graf@amazon.com>
> Link: https://www.qemu.org/docs/master/specs/vmgenid.html
> Link: https://learn.microsoft.com/en-us/windows/win32/hyperv_v2/virtual-machine-generation-identifier
> Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
> ---
>   .../bindings/rng/microsoft,vmgenid.yaml       | 49 +++++++++++++++++++
>   MAINTAINERS                                   |  1 +
>   2 files changed, 50 insertions(+)
>   create mode 100644 Documentation/devicetree/bindings/rng/microsoft,vmgenid.yaml
> 
> diff --git a/Documentation/devicetree/bindings/rng/microsoft,vmgenid.yaml b/Documentation/devicetree/bindings/rng/microsoft,vmgenid.yaml
> new file mode 100644
> index 000000000000..8f20dee93e7e
> --- /dev/null
> +++ b/Documentation/devicetree/bindings/rng/microsoft,vmgenid.yaml
> @@ -0,0 +1,49 @@
> +# SPDX-License-Identifier: (GPL-2.0-only OR BSD-2-Clause)
> +%YAML 1.2
> +---
> +$id: http://devicetree.org/schemas/rng/microsoft,vmgenid.yaml#
> +$schema: http://devicetree.org/meta-schemas/core.yaml#
> +
> +title: Virtual Machine Generation ID
> +
> +maintainers:
> +  - Jason A. Donenfeld <Jason@zx2c4.com>
> +
> +description:
> +  Firmwares or hypervisors can use this devicetree to describe an
> +  interrupt and a shared resource to inject a Virtual Machine Generation ID.
> +  Virtual Machine Generation ID is a globally unique identifier (GUID) and
> +  the devicetree binding follows VMGenID specification defined in
> +  http://go.microsoft.com/fwlink/?LinkId=260709.
> +
> +properties:
> +  compatible:
> +    const: microsoft,vmgenid
> +
> +  reg:
> +    description:
> +      Specifies a 16-byte VMGenID in endianness-agnostic hexadecimal format.
> +    maxItems: 1
> +
> +  interrupts:
> +    description:
> +      Interrupt used to notify that a new VMGenID is available.
> +    maxItems: 1
> +
> +required:
> +  - compatible
> +  - reg
> +  - interrupts
> +
> +additionalProperties: false
> +
> +examples:
> +  - |
> +    #include <dt-bindings/interrupt-controller/arm-gic.h>
> +    rng@80000000 {
> +      compatible = "microsoft,vmgenid";
> +      reg = <0x80000000 0x1000>;
> +      interrupts = <GIC_SPI 35 IRQ_TYPE_EDGE_RISING>;
> +    };
> +
> +...
> diff --git a/MAINTAINERS b/MAINTAINERS
> index aa3b947fb080..c84ac9258a48 100644
> --- a/MAINTAINERS
> +++ b/MAINTAINERS
> @@ -18463,6 +18463,7 @@ M:      "Theodore Ts'o" <tytso@mit.edu>
>   M:     Jason A. Donenfeld <Jason@zx2c4.com>
>   S:     Maintained
>   T:     git https://git.kernel.org/pub/scm/linux/kernel/git/crng/random.git
> +F:     Documentation/devicetree/bindings/rng/microsoft,vmgenid.yaml
>   F:     drivers/char/random.c
>   F:     drivers/virt/vmgenid.c
> 
> --
> 2.44.0
> 
Since I am on leave, looping in Babis to review/verify the patches.