Re: [RFC] Virtualizing tagged disaggregated memory capacity (app specific, multi host shared)

Gregory Price
2024-10-22T14:11:58Z

On Tue, Sep 17, 2024 at 07:37:21PM +0000, Jonathan Cameron wrote:
> > * Said mechanism should not be explicitly CXL-specific.
>
> Somewhat agreed, but I don't want to invent a new spec just to avoid explicit
> ties to CXL. I'm not against using CXL to present HBM / ACPI Specific Purpose
> memory for example to a VM. It will trivially work if that is what a user
> wants to do and also illustrates that this stuff doesn't necessarily just
> apply to capacity on a memory pool - it might just be 'weird' memory on the host.
>

I suspect if you took all the DCD components of the current CXL device
and repackaged it into a device called "DefinitelyNotACXLDCDDevice", that
the CXL device inherited, this whole discussion goes away.

Patches welcome? :]

> > * Finding a tagged capacity devdax device in a VM should work the same as it
> > does running on bare metal.
>
> Absolutely - that's a requirement.
>
> > * The file-backed (and devdax-backed) devdax abstraction is needed in qemu.
>
> Maybe. I'm not convinced the abstraction is needed at that particular level.
>
> > * Beyond that, I'm not yet sure what the lookup mechanism should be. Extra
> > points for being easy to implement in both physical and virtual systems.
>
> For physical systems we aren't going to get agreement :( For the systems
> I have visibility of there will be some diversity in hardware, but the
> presentation to userspace and up consistency should be doable.
>
> Jonathan
>
> >
> > Thanks for teeing this up!
> > John
> >
> >
> > [1] https://github.com/cxl-micron-reskit/famfs/blob/master/README.md
> >
>
>