Re: [PATCH v4] virt: vmgenid: introduce driver for reinitializing RNG on VM fork

[Alexander Graf <graf@amazon.com>]

On 25.02.22 16:34, Jason A. Donenfeld wrote:
> On Fri, Feb 25, 2022 at 04:16:27PM +0100, Ard Biesheuvel wrote:
>>>> I just booted up a Windows VM, and it looks like Hyper-V uses
>>>> "Hyper_V_Gen_Counter_V1", which is also quite long, so we can't really
>>>> HID match on that either.
>>>
>>> Yes, due to the same problem. I'd really prefer we sort out the ACPI
>>> matching before this goes mainline. Matching on _HID is explicitly
>>> discouraged in the VMGenID spec.
>>>
>> OK, this really sucks. Quoting the ACPI spec:
>>
>> """
>> A _HID object evaluates to either a numeric 32-bit compressed EISA
>> type ID or a string. If a string, the format must be an alphanumeric
>> PNP or ACPI ID with no asterisk or other leading characters.
>> A valid PNP ID must be of the form "AAA####" where A is an uppercase
>> letter and # is a hex digit.
>> A valid ACPI ID must be of the form "NNNN####" where N is an uppercase
>> letter or a digit ('0'-'9') and # is a hex digit. This specification
>> reserves the string "ACPI" for use only with devices defined herein.
>> It further reserves all strings representing 4 HEX digits for
>> exclusive use with PCI-assigned Vendor IDs.
>> """
>>
>> So now we have to implement Microsoft's fork of ACPI to be able to use
>> this device, even if we expose it from QEMU instead of Hyper-V? I
>> strongly object to that.
>>
>> Instead, we can match on _HID exposed by QEMU, and cordially invite
>> Microsoft to align their spec with the ACPI spec.
> I don't know about that... Seems a bit extreme. Hopefully Alex will be
> able to sort something out with the ACPI people, and this driver will
> work inside of Hyper-V.
>
> Here's what we currently have:
>
>    static const struct acpi_device_id vmgenid_ids[] = {
>      { "VMGENID", 0 },  <------------------------------------ ???
>      { "QEMUVGID", 0 }, <------------------------------------ QEMU
>      { },
>    };
>
> Adrian added "VMGENID" in last year's v4, so I copied that for this new
> driver here. But does anybody know which hypervisor it is for? Some
> internal Amazon thing? Firecracker? VMware? In case Alex does not
> succeed with the ACPI changes, it'd be nice to know which HIDs for
> which hypervisors we do and do not support.


I believe "VMGENID" was for the firecracker prototype that Adrian built 
back then, yeah. Matching on _HID for this is a rat hole unfortunately, 
so let's see what the ACPI patch gets us :).


Alex





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