Re: [BUG] hw/i386/pc.c: CXL Fixed Memory Window should not reserve e820 in bios

[Igor Mammedov <imammedo@redhat.com>]

On Tue, 8 Nov 2022 12:21:11 +0100
Gerd Hoffmann <kraxel@redhat.com> wrote:

> > >> > diff --git a/hw/i386/pc.c b/hw/i386/pc.c
> > >> > index 566accf7e6..5bf5465a21 100644
> > >> > --- a/hw/i386/pc.c
> > >> > +++ b/hw/i386/pc.c
> > >> > @@ -1061,7 +1061,6 @@ void pc_memory_init(PCMachineState *pcms,
> > >> >          hwaddr cxl_size = MiB;
> > >> >
> > >> >          cxl_base = pc_get_cxl_range_start(pcms);
> > >> > -        e820_add_entry(cxl_base, cxl_size, E820_RESERVED);  
> 
> Just dropping it doesn't look like a good plan to me.
> 
> You can try set etc/reserved-memory-end fw_cfg file instead.  Firmware
> (both seabios and ovmf) read it and will make sure the 64bit pci mmio
> window is placed above that address, i.e. this effectively reserves
> address space.  Right now used by memory hotplug code, but should work
> for cxl too I think (disclaimer: don't know much about cxl ...).

As far as I know CXL impl. in QEMU isn't using etc/reserved-memory-end
at all, it' has its own mapping.

Regardless of that, reserved E820 entries look wrong, and looking at
commit message OS is right to bailout on them (expected according
to ACPI spec).
Also spec says 

"
E820 Assumptions and Limitations
 [...]
 The platform boot firmware does not return a range description for the memory mapping of
 PCI devices, ISA Option ROMs, and ISA Plug and Play cards because the OS has mechanisms
 available to detect them.
"

so dropping reserved entries looks reasonable from ACPI spec point of view.
(disclaimer: don't know much about cxl ... either)
> 
> take care & HTH,
>   Gerd
>