Re: [PATCH v1 0/4] virt: vmgenid: Add devicetree bindings support

From: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
To: "Landge, Sudan" <sudanl@amazon.co.uk>,
	Rob Herring <robh@kernel.org>,
	David Woodhouse <dwmw2@infradead.org>
Cc: 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>
Subject: Re: [PATCH v1 0/4] virt: vmgenid: Add devicetree bindings support
Date: Fri, 22 Mar 2024 06:40:43 +0100	[thread overview]
Message-ID: <17611183-f288-47fe-a5e1-91ee16168db0@linaro.org> (raw)
In-Reply-To: <db5a1027-93b7-4630-b679-8a654905dc48@amazon.co.uk>

On 21/03/2024 18:39, Landge, Sudan wrote:
> 
> 
> On 21/03/2024 13:32, Rob Herring 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.
>>
>>
>>
>> On Wed, Mar 20, 2024 at 04:55:45PM +0000, David Woodhouse wrote:
>>> 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 support for
>>>>>>> the Virtual Machine Generation ID (vmgenid) driver.
>>>>>>>
>>>>>>> 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.
>>>>>>> We would like to extend vmgenid to support devicetree bindings because:
>>>>>>> 1. A device should not be defined as an ACPI or DT only device.
>>>>
>>>> This (and the binding patch) tells me nothing about what "Virtual
>>>> Machine Generation ID driver" is and isn't really justification for
>>>> "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.
>>
>> That doc mentions fw_cfg for which we already have a binding. Why can't
>> it be used/extended here?
> QEMU has support for vmgenid but even they do not pass vmgenid directly 
> to the guest kernel using fw_cfg. QEMU passes the vmgenid/UUID via 
> fw_cfg to an intermediate UEFI firmware. This UEFI firmware, running as 
> a guest in QEMU, reads the UUID from fw_cfg and creates ACPI tables for 
> it. The UEFI firmware then passes the UUID information to the guest 
> kernel via ACPI.
> This approach requires yet another intermediary which is UEFI firmware 
> and adds more complexity than ACPI for minimalist hypervisors that do 
> not have an intermediate bootloader today.

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 QEMU.

Best regards,
Krzysztof