Re: [ACPI Code First ECN v2]: Generic Port, performace data for hotplug memory

[Dan Williams <dan.j.williams@intel.com>]

On Mon, Apr 19, 2021 at 3:56 PM Vikram Sethi <vsethi@nvidia.com> wrote:
[..]
> > * Replace all instances of "Initiator" with "Initiator / Port" in "Table
> >   5.59 Flags - Generic Initiator Affinity Structure", including the
> >   table name.
>
> I wanted to discuss the implications of a CXL host bridge implementation that
> does not set the "Architectural Transactions" bit/flag in the aforementioned
> Flags in Table 5.59.
>
> Since the kernel would be expecting all "System RAM" to have equivalent
> Functional properties, if HDM cannot have all the same functionality, then in
> the absence of ISA specific ACPI tables clarifying what architectural feature isn't
> supported, the kernel may be forced to not online the HDM memory as system
> RAM. If there is more granular expression of what features are lacking in a ISA
> Specific table (eg Memory Tagging/Memory Protection keys not supported),
> the kernel could choose to not enable that feature in all of system RAM (if
> discrepancy discovered at boot), but still online the HDM as System RAM.
>
> To that end, it may be useful to clarify this to host vendors by way of an
> Implementation note (ideally in the CXL specification). Something like:
> "CXL hosts are encouraged to support all architectural features in HDM
> as they do in CPU attached memory to avoid either the memory from
> being onlined as System RAM, or the architectural feature being disabled.
> Hosts must indicate architectural parity for HDM in ACPI SRAT
> “Generic Port” flags “Architectural transactions” bit by setting it to 1.
> A port that sets this bit to 0 will need ISA specific ways/ACPI tables to
> describe which specific ISA features would not work in HDM, so an OS
> could disable that memory or that feature."
>
> Thoughts?

The problem, as you know, is that those features are already defined
without those "ISA specific ways / ACPI tables". I think it simply
must be the case that the only agent in the system that is aware of
the intersection of capabilities between ISA and CXL (platform
firmware) must mitigate the conflict. I.e. so that the CXL
infrastructure need not worry about ISA feature capability and vice
versa. To me, this looks like a platform firmware pre-boot
configuration menu / switch that turns off CXL (declines to publish
ACPI0016 devices) if incompatible ISA feature "X" is enabled, or the
reverse turns off ISA feature "X" if CXL is enabled. In other words,
the conflict needs to be resolved before the OS boots, setting this
bit to 0 is not a viable option for mitigating the conflict because
there is no requirement for the OS to even look at this flag.