Re: [Qemu-devel] [RFC v5 86/86] 440fx: fix PAM, PCI holes

Anthony Liguori
2011-07-25T13:07:37Z

On 07/20/2011 11:50 AM, Avi Kivity wrote:
> The current implementation of PAM and the PCI holes is broken in several
> ways:
>
> - PCI BARs are not restricted to the PCI hole (a BAR may hide memory)

Technically, a BAR can be mapped to any non-RAM memory location.

> - PCI devices do not respect PAM (if a PCI device maps a region while
> PAM maps the region to RAM, the request will be honored)

I assume you mean SMM shadowing, right? PAM doesn't cover an area
that's ever forwarded to the PCI bus.

> This patch fixes things by introducing a pci address space, and using
> memory region aliases to represent PAM regions, SMRAM, and PCI holes.
>
> The memory hierarchy looks something like
>
> system_memory
> |
> +--- low memory alias (0-0xe0000000)

According to the spec, PCI memory doesn't start at e00... but rather at
the top of RAM. In fact, this is what the spec says:

"The address range from the top of main DRAM to 4 Gbytes (top of
physical memory space supported by the 440FX PCIset) is normally mapped
to PCI. The PMC forwards all accesses within this address range to PCI.
There are two sub-ranges within this address range defined as APIC
Configuration Space and High BIOS Address Range."

So the right thing to do is to forward all accesses from
low_memory_memsize ... 4GB to the PCI bus.

Regards,

Anthony Liguori