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

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Date: Fri, 22 Mar 2024 06:40:43 +0100
From: Krzysztof Kozlowski <krzysztof.kozlowski@...aro.org>
To: "Landge, Sudan" <sudanl@...zon.co.uk>, Rob Herring <robh@...nel.org>,
 David Woodhouse <dwmw2@...radead.org>
Cc: Sudan Landge <sudanl@...zon.com>, tytso@....edu, Jason@...c4.com,
 krzysztof.kozlowski+dt@...aro.org, conor+dt@...nel.org,
 sathyanarayanan.kuppuswamy@...ux.intel.com, thomas.lendacky@....com,
 dan.j.williams@...el.com, devicetree@...r.kernel.org,
 linux-kernel@...r.kernel.org, graf@...zon.de, bchalios@...zon.es,
 xmarcalx@...zon.co.uk, ardb@...nel.org, benh <benh@...nel.crashing.org>
Subject: Re: [PATCH v1 0/4] virt: vmgenid: Add devicetree bindings support

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