Re: RFC: x86 memory map, where to put CXL ranges?

[Alex Bennée]

Jonathan Cameron <Jonathan.Cameron@Huawei.com> writes:

> Hi All,
>
> For CXL emulation we require a couple of types of memory range that
> are then provided to the OS via the CEDT ACPI table.
>
> 1) CXL Host Bridge Structures point to CXL Host Bridge Component Registers.
> Small regions for each CXL Host bridge that are mapped into the memory space.
> 64k each.  In theory we may have a huge number of these but in reality I
> think 16 will do for any reasonable system.
>
> 2) CXL Fixed Memory Window Structures (CFMWS)
> Large PA space ranges (multiple TB) to which various CXL devices can be 
> assigned
> and their address decoders appropriately programmed.
> Each such CFMWS will have particular characteristics such as interleaving 
> across
> multiple host bridges.  The can potentially be huge but are a system
> characteristic.  For emulation purposes it won't matter if they move around
> dependent on what else is the machine has configured. So I'd like to
> just configure their size rather than fully specify them at the command line
> and possibly clash on PA space with something else.  Alternatively could
> leave them as fully specified at the command line (address and size) and just
> error out if the hit memory already in use for something else.
>
> Now unfortunately there are no systems out there yet that we can just
> copy the memory map from...
>
> Coming form an Arm background I have only a vague idea of how this should be
> done for x86 so apologies if it is a stupid question.
>
> My current approach is to put these above device_memory and moving
> the pci hole up appropriately.

Which board model would be be talking about here? virt? Or maybe we need
a new one?

If it's virt I would look at extended_memmap which floats above the
configured RAM size and means less shuffling around of the relatively
crowded lower address space.

I have no idea about how this is handled on x86 though.

> Is that the right choice?
>
> On Arm I currently have the Host Bridge Structures low down in the MemMap and 
> the CFMWS
> can go above the device memory.  Comments on that also welcome.
>
> In Ben's RFC the host bridge component register location was marked as a TODO
> and a arbitrary address used in the meantime so time to figure out how to 
> clean
> that up.
>
> Thanks,
>
> Jonathan


-- 
Alex Bennée