Hi Alex,
On Fri, Feb 25, 2022 at 02:57:38PM +0100, Alexander Graf wrote:
> > +static const struct acpi_device_id vmgenid_ids[] = {
> > + { "VMGENID", 0 },
> > + { "QEMUVGID", 0 },
>
>
> According to the VMGenID spec[1], you can only rely on _CID and _DDN for
> matching. They both contain "VM_Gen_Counter". The list above contains
> _HID values which are not an official identifier for the VMGenID device.
>
> IIRC the ACPI device match logic does match _CID in addition to _HID.
> However, it is limited to 8 characters. Let me paste an experimental
> hack I did back then to do the _CID matching instead.
>
> [1]
> https://download.microsoft.com/download/3/1/C/31CFC307-98CA-4CA5-914C-D9772691E214/VirtualMachineGenerationID.docx
>
>
> Alex
>
> diff --git a/drivers/acpi/bus.c b/drivers/acpi/bus.c
> index 1682f8b454a2..452443d79d87 100644
> --- a/drivers/acpi/bus.c
> +++ b/drivers/acpi/bus.c
> @@ -748,7 +748,7 @@ static bool __acpi_match_device(struct acpi_device
> *device,
> /* First, check the ACPI/PNP IDs provided by the caller. */
> if (acpi_ids) {
> for (id = acpi_ids; id->id[0] || id->cls; id++) {
> - if (id->id[0] && !strcmp((char *)id->id, hwid->id))
> + if (id->id[0] && !strncmp((char *)id->id, hwid->id,
> ACPI_ID_LEN - 1))
> goto out_acpi_match;
> if (id->cls && __acpi_match_device_cls(id, hwid))
> goto out_acpi_match;
> diff --git a/drivers/virt/vmgenid.c b/drivers/virt/vmgenid.c
> index 75a787da8aad..0bfa422cf094 100644
> --- a/drivers/virt/vmgenid.c
> +++ b/drivers/virt/vmgenid.c
> @@ -356,7 +356,8 @@ static void vmgenid_acpi_notify(struct acpi_device
> *device, u32 event)
> }
>
> static const struct acpi_device_id vmgenid_ids[] = {
> - {"QEMUVGID", 0},
> + /* This really is VM_Gen_Counter, but we can only match 8 characters */
> + {"VM_GEN_C", 0},
> {"", 0},
> };
I recall this part of the old thread. From what I understood, using
"VMGENID" + "QEMUVGID" worked /well enough/, even if that wasn't
technically in-spec. Ard noted that relying on _CID like that is
technically an ACPI spec notification. So we're between one spec and
another, basically, and doing "VMGENID" + "QEMUVGID" requires fewer
changes, as mentioned, appears to work fine in my testing.
However, with that said, I think supporting this via "VM_Gen_Counter"
would be a better eventual thing to do, but will require acks and
changes from the ACPI maintainers. Do you think you could prepare your
patch proposal above as something on-top of my tree [1]? And if you can
convince the ACPI maintainers that that's okay, then I'll happily take
the patch.
Jason
[1] https://git.kernel.org/pub/scm/linux/kernel/git/crng/random.git/
