Re: [PATCH] hw/mem: support zero memory size CXL device

[Gregory Price <gourry@gourry.net>]

On Tue, Dec 03, 2024 at 09:15:51PM +0000, Hongjian Fan wrote:
> Hi Jonathan,
> 
> I'm trying to emulate our memory appliance which is similar to a MH-SLD. The memory device is connected to the host server while the size of the memory could be changed by the out-of-band fabric manager. If there is no memory assigned to the host, the CXL device will be booted as zero memory size.

This should not be how this is done.

The ACPI tables should report the maximum possible size, and the DCD
infrastructure should enable physical regions that have been added to the host.

Changing ACPI tables to report 0 memory size will basically result
in the host memory map not reserving physical memory regions for that
device.

See this emulation example of an MHSLD - which can be used for DCD.

https://lore.kernel.org/linux-cxl/20241018161252.8896-1-gourry@gourry.net/

> Recently we got some interest on trying our fabric manager software without having the real hardware. Supporting zero memory size in QEMU will be needed in this scenario.
> Some detail about our memory appliance could be found from our OCP presentation: https://drive.google.com/file/d/1i8kBsBfRGjNqnTQqJ9upC-Xm9o56Y2Y5/view?usp=drive_link
> 
> Thanks,
> Hongjian Fan
> 
> 
> Seagate Internal