Re: [Qemu-devel] [edk2] Windows does not support DataTableRegion at all [was: docs: describe QEMU's VMGenID design]

[Laszlo Ersek]

On 09/14/15 23:12, Bill Paul wrote:

[snip -- see my other email too]

> Also, if you want to talk business cases, I'm assuming that somewhere 
> Microsoft claims ACPI compliance.

Correct; this page:

<https://msdn.microsoft.com/en-us/library/windows/hardware/dn551195%28v=vs.85%29.aspx>

states

    Version 5.0 of the Microsoft ACPI source language (ASL) compiler
    supports the features in the Advanced Configuration and Power
    Interface Specification, Revision 5.0 [...]

And the separately downloadable ASL.exe that I tried starts with a banner that 
claims ACPI 4.0 compliance:

> C:\Program Files (x86)\Microsoft ASL Compiler v4.0>asl.exe x.dsl
> Microsoft ACPI Source Language Assembler Version 4.0.0NT [Aug 28 2009, 
> 18:36:36]
> 
> Copyright (c) 1996,2009 Microsoft Corporation
> Compliant with the ACPI 4.0 Specification
> 
> [...]

The DataTableRegion() operator is from ACPI 2.0.

In ACPI 6.0 (the most recent release), it is still there.

(And, logically, if you can compile DataTableRegion() from ASL to AML (-> 
DefDataRegion), then your AML interpreter should also be able to parse and 
execute the binary DefDataRegion encoding codified by the standard.)

> If so, then clearly that claim is untrue,

Let's say, "not precise".

I think such gaps in support for various industry standards are not uncommon, 
but I believe they should be publicly documented. Using Google, I couldn't find 
a trace of this limitation. If there had been public documentation about this 
(or maybe a public bug tracker, or just a tech support forum post...), it would 
have saved us many many hours, at minimum.

(At this point though, the best for us would be if Microsoft decided to 
implement DataTableRegion() in ACPI.SYS, and push it out as KBxxxxx.)
 
> and I'd be willing to bet this isn't the only place where their 
> implementation 
> deviates from the spec either.

I assume that's likely, yes.

> It's bad for business to claim compliance with 
> an industry standard and then very obviously (and maybe even deliberately) 
> not 
> comply with it.

The inaccurate claim about compliance is certainly confusing.

Establishing the non-compliance (from the oustide anyway) was anything but 
obvious. But, now that we've seen the evidence, it's quite factual.

> (If, as you say, this has already been reported and Microsoft 
> decided not to fix it, then it's no longer just a good faith mistake.)

I didn't try to state that as a fact, I just opined that in the 15 years since 
the release of the ACPI 2.0 revision, someone must surely have tried 
DataTableRegion(). Assuming that programmer worked for a big BIOS company 
(which I think is a safe assumption -- before virtualization has become 
commonplace, who else looked into *writing* ACPI tables?), it seems to follow 
that a bug would be reported in some way.

> It's 
> even worse to do that while also being a prominent member of the very same 
> industry committee responsible for defining the standard in the first place.

Right, it's strange.

> In any case, bugs are bugs. You shouldn't need a justification to fix them, 
> especially with iron-clad proof of their existence like you have here.

Bugfixing has costs. That's what I'm worried about a little bit. I don't know 
what to put in the other side of the balance. "Improving compliance" should 
have marketing value, minimally. Then, "helping QEMU developers implement 
Microsoft's VMGENID spec, thereby improving Windows guest utility on QEMU" 
should ultimately translate to wider Windows guest adoption.

> All that aside, I don't know who to report it to either. Maybe this is a good 
> time to establish a way to do that, because I doubt this will be the last 
> time 
> it will be necessary.

I'm hopeful about the ASWG connection.

Thanks!
Laszlo

> -Bill
> 
>> Thanks
>> Laszlo
>>
>>> -Bill
>>>
>>>>> I suggest we go back to the last Gal's series
>>>>> which is though not universal but a simple and
>>>>> straightforward solution.
>>>>> That series with comments addressed probably
>>>>> is what we need in the end.
>>>>
>>>> I agree (I commented the same on the RHBZ too). The only one requirement
>>>> we might not satisfy with that is that the page containing the
>>>> generation ID must always be mapped as cacheable. In practice it doesn't
>>>> seem to cause issues.
>>>>
>>>> We tried to play nice, but given that (a) the vmgenid doc doesn't
>>>> mention a real requirement about the _CRS -- ie. no IO descriptors are
>>>> allowed to be in it --, (b) Windows doesn't support DataTableRegion(), I
>>>> doubt we could cover our bases 100% anyway. There can be any number of
>>>> further hidden requirements, and hidden gaps in ACPI support too, so
>>>> it's just business as usual with Windows: whatever works, works, don't
>>>> ask why.
>>>>
>>>> Just my opinion of course.
>>>>
>>>> Laszlo
>>>>
>>>>>> The only crazy thing you didn't try is to use
>>>>>> an XSDT instead of the DSDT.
>>>>>> I find it unlikely that this will help ...
>>>>
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>