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

[Jonathan Cameron]

On Tue, 10 Dec 2024 14:13:29 -0500
Gregory Price <gourry@gourry.net> wrote:

> 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.
Agreed, but...

It sounds like a pre DCD boot time only pooling solution?

What is the path to adding memory?

> 
> 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.
This isn't the ACPI bit, it's just the device reporting.  Can have a huge
CFMWS and tiny devices.

> 
> 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

That doesn't really give me any indication of how the addition of memory 
happens.

Jonathan

> > 
> > Thanks,
> > Hongjian Fan
> > 
> > 
> > Seagate Internal