Re: [PATCH] ptp: Add vDSO-style vmclock support

[Michael S. Tsirkin]

On Fri, Jul 26, 2024 at 07:28:28PM +0100, David Woodhouse wrote:
> On 26 July 2024 17:49:58 BST, Jonathan Cameron <Jonathan.Cameron@xxxxxxxxxx> wrote:
> >On Thu, 25 Jul 2024 14:50:50 +0100
> >David Woodhouse <dwmw2@xxxxxxxxxxxxx> wrote:
> >
> >> On Thu, 2024-07-25 at 08:33 -0400, Michael S. Tsirkin wrote:
> >> > On Thu, Jul 25, 2024 at 01:31:19PM +0100, David Woodhouse wrote:  
> >> > > On Thu, 2024-07-25 at 08:29 -0400, Michael S. Tsirkin wrote:  
> >> > > > On Thu, Jul 25, 2024 at 01:27:49PM +0100, David Woodhouse wrote:  
> >> > > > > On Thu, 2024-07-25 at 08:17 -0400, Michael S. Tsirkin wrote:  
> >> > > > > > On Thu, Jul 25, 2024 at 10:56:05AM +0100, David Woodhouse wrote:  
> >> > > > > > > > Do you want to just help complete virtio-rtc then? Would be easier than
> >> > > > > > > > trying to keep two specs in sync.  
> >> > > > > > > 
> >> > > > > > > The ACPI version is much more lightweight and doesn't take up a
> >> > > > > > > valuable PCI slot#. (I know, you can do virtio without PCI but that's
> >> > > > > > > complex in other ways).
> >> > > > > > >   
> >> > > > > > 
> >> > > > > > Hmm, should we support virtio over ACPI? Just asking.  
> >> > > > > 
> >> > > > > Given that we support virtio DT bindings, and the ACPI "PRP0001" device
> >> > > > > exists with a DSM method which literally returns DT properties,
> >> > > > > including such properties as "compatible=virtio,mmio" ... do we
> >> > > > > already?
> >> > > > > 
> >> > > > >   
> >> > > > 
> >> > > > In a sense, but you are saying that is too complex?
> >> > > > Can you elaborate?  
> >> > > 
> >> > > No, I think it's fine. I encourage the use of the PRP0001 device to
> >> > > expose DT devices through ACPI. I was just reminding you of its
> >> > > existence.  
> >> > 
> >> > Confused. You said "I know, you can do virtio without PCI but that's
> >> > complex in other ways" as the explanation why you are doing a custom
> >> > protocol.  
> >> 
> >> Ah, apologies, I wasn't thinking that far back in the conversation.
> >> 
> >> If we wanted to support virtio over ACPI, I think PRP0001 can be made
> >> to work and isn't too complex (even though it probably doesn't yet work
> >> out of the box).
> >> 
> >> But for the VMCLOCK thing, yes, the simple ACPI device is a lot simpler
> >> than virtio-rtc and much more attractive.
> >> 
> >> Even if the virtio-rtc specification were official today, and I was
> >> able to expose it via PCI, I probably wouldn't do it that way. There's
> >> just far more in virtio-rtc than we need; the simple shared memory
> >> region is perfectly sufficient for most needs, and especially ours.
> >> 
> >> I have reworked
> >> https://git.infradead.org/users/dwmw2/linux.git/shortlog/refs/heads/vmclock
> >> to take your other feedback into account.
> >> 
> >> It's now more flexible about the size handling, and explicitly checking
> >> that specific fields are present before using them. 
> >> 
> >> I think I'm going to add a method on the ACPI device to enable the
> >> precise clock information. I haven't done that in the driver yet; it
> >> still just consumes the precise clock information if it happens to be
> >> present already. The enable method can be added in a compatible fashion
> >> (the failure mode is that guests which don't invoke this method when
> >> the hypervisor needs them to will see only the disruption signal and
> >> not precise time).
> >> 
> >> For the HID I'm going to use AMZNVCLK. I had used QEMUVCLK in the QEMU
> >> patches, but I'll change that to use AMZNVCLK too when I repost the
> >> QEMU patch.
> >
> >That doesn't fit with ACPI _HID definitions.
> >Second set 4 characters need to be hex digits as this is an
> >ACPI style ID (which I assume this is given AMZN is a valid
> >vendor ID.  6.1.5 in ACPI v6.5
> >
> >Maybe I'm missing something...
> >
> >J
> >
> >
> 
> 
> 
> Hm, is the same not true for QEMUVGID and AMZNVGID, which I was using as an example?
> 
> QEMU seemed to get to 0002, and AFAICT the VMGENID patches were initially posted using QEMU0003, but what's actually in QEMU now is QEMUVGID. So I presumed that was now the preferred option.

Glad you asked :)


ACPI 1.0 indeed did not place restrictions on it:

6.1.4 _HID
This object is used to supply the OS with the device’s Plug and Play Hardware ID. When
describing a platform, use of any _HID objects is optional. However, a _HID object must
be used to describe any device that will be enumerated by the ACPI driver. The ACPI
driver only enumerates a device when no bus enumerator can detect the device ID. For
example, devices on an ISA bus are enumerated by the ACPI driver. Use the _ADR
object to describe devices enumerated by bus enumerators other than the ACPI driver.
A _HID object evaluates to either a numeric 32-bit compressed EISA type ID or a string.


Then 3.0 was very draconic:
6.1.4 _HID (Hardware ID)
This object is used to supply OSPM with the device’s Plug and Play hardware ID.8 When describing a
platform, use of any _HID objects is optional. However, a _HID object must be used to describe any device
that will be enumerated by OSPM. OSPM only enumerates a device when no bus enumerator can detect the
device ID. For example, devices on an ISA bus are enumerated by OSPM. Use the _ADR object to describe
devices enumerated by bus enumerators other than OSPM.
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 “ACPI####” where # is a hex digit.

Then 5.0 changed it to:

6.1.5 _HID (Hardware ID)
This object is used to supply OSPM with the device’s Plug and Play hardware ID.1
1.
256
A Plug and Play ID or ACPI ID can be obtained by sending e-mail to pnpid@xxxxxxxxxxxxx.
Hewlett-Packard/Intel/Microsoft/Phoenix/ToshibaAdvanced Configuration and Power Interface Specification
When describing a platform, use of any _HID objects is optional. However, a _HID object must be
used to describe any device that will be enumerated by OSPM. OSPM only enumerates a device
when no bus enumerator can detect the device ID. For example, devices on an ISA bus are
enumerated by OSPM. Use the _ADR object to describe devices enumerated by bus enumerators
other than OSPM.
Arguments:
None
Return Value:
An Integer or String containing the HID
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.



Long story short, QEMUVGID is indeed out of spec, but it works
both because of guest compatibility with ACPI 1.0, and because no one
much uses it.


-- 
MST