Re: [PATCH] db: Add Intel NMI-source reporting feature flag

[Andrew Cooper <andrew.cooper3@citrix.com>]

On 26/09/2024 2:12 pm, Ahmed S. Darwish wrote:
> Hi Sohil,
>
> On Mon, 23 Sep 2024, Sohil Mehta wrote:
>>>  <bit21 len="1"  id="amx_fp16"   desc="AMX-FP16: FP16 tile operations">
>>>    <linux        feature="true"  proc="false" />
>> I couldn't find much documentation for the <linux> tag. When is it
>> expected to be included?
>>
>> Also, are the following linux hint attributes documented somewhere? I
>> couldn't decipher them from their name itself.
>>
>>>   <xs:complexType name="linuxType">
>>>     <xs:attribute type="xs:boolean" name="feature" use="required" />
>>>     <xs:attribute type="xs:boolean" name="proc"    use="required" />
>>>     <xs:attribute type="xs:string"  name="altid"   use="optional" />
>>>     <xs:attribute type="xs:string"  name="procid"  use="optional" />
>>>   </xs:complexType>
>>>
> Yeah, I should've documented the schema more, either in-place or in a
> separate file.
>
> The <linux> tag means that this bit is also represented inside the Linux
> Kernel x86 tree in various means.
>
> Here are two nice examples from leaf_01.xml:
>
>   <bit2  len="1" id="dtes64"    desc="64-bit DS save area">
>     <vendors>
>       <vendor>Intel</vendor>
>     </vendors>
>     <linux	 feature="true"  proc="true" />
>     <xen         feature="true"  attrs="" />
>   </bit2>
>   ...
>   <bit0  len="1" id="sse3"      desc="Streaming SIMD Extensions 3 (SSE3)">
>     <vendors>
>       <vendor>Intel</vendor>
>       <vendor>AMD</vendor>
>     </vendors>
>     <linux       feature="true"	proc="true" altid="xmm3" procid="pni" />
>     <xen         feature="true"	attrs="A" />
>   </bit0>
>
> <linux> "feature=true" attribute means that the feature bit is also an
> X86_FEATURE_* flag.  So, in the "dtes64" case, there's indeed an
> X86_FEATURE_DTES64 defined flag in the Linux kernel x86 tree.
>
> <linux> "feature=true altid=NAME" means there's an X86_FEATURE_* flag,
> but the name is slightly different from the database attribute id name
> (for various reasons).  Thus, in the SSE3 feature flag case, the Linux
> feature flag is actually called X86_FEATURE_XMM3 (altid="xmm3".)
>
> <linux> "proc=true" means that the feature bit is also displayed at
> Linux's /proc/cpuinfo if available on the CPU.  So, in the "dtes64"
> case, you'll indeed see dtes64 flag at /proc/cpuinfo on most Intel CPUs.
>
> <linux> "proc=true procid=PROCNAME' means that the feature bit is
> displayed at /proc/cpuinfo, but the flag name there is different from
> the database attribute id name (for various reasons.)  Thus, in the
> "sse3" case, "procid=pni" because that's how Linux displays SSE3 feature
> flag at /proc/cpuinfo.
>
> So far, that <linux> tag is not used by any of the XSLT transformers.
> But, we plan to use this information as we go deeper into the Linux x86
> tree refactoring(s).  For now, the goal was to cover all known names for
> the cpuid feature bits, especially when they differ; i.e. canonical name
> vs. Linux X86_FEATURE name vs. Linux /proc/cpuinfo name.
>
> Thanks for the question! I'll indeed need to document that part of the
> schema before the v2.0 release.

I guess I need to start getting back to this...

The Xen 'attrs' need stripping. More specifically, they need to live in
Xen itself, because they are different by release, and are used in the
derivation of what is safe to offer to guests.

But the same is true for 'feature' too.  It's one think to know what the
bit means, but it's something else to be making a statement about what
is supported in an external project.

When this is used in earnest, regenerating the headers will be needed in
each stable tree to get e.g. the new speculation (anti)features, and
doing so needs to not change the other features="true" for bits only
supported in newer branches.

~Andrew