Re: [RFC] - Mapping ACPI tables as CACHED

[Len Brown <lenb@kernel.org>]

On Tue, 17 Aug 2010, Jack Steiner wrote:

> On EFI-enabled systems (like UV), the ACPI tables are already mapped as
> WB memory. This is done in the EFI function efi_enter_virtual_mode().
> 
> The E820_ACPI & E820_NVS regions will be mapped as WB memory if the BIOS
> sets the WB attribute in the EFI memmap entry for the chunk of memory.
> 
> The ACPI code in acpi_os_map_memory() is not currently aware of the EFI mapping
> & currently maps the memory as UC. This seems like a bug.
> 
> In order to prevent attribute aliasing, I think the ACPI mapping needs to
> be consistent with the EFI mapping.  Otherwise, we will have the OS
> referencing the memory as UC at the same time BIOS is referencing it as WB.

When would the firmware touch those tables after booting the OS?

We once found a Toshiba laptop where the BIOS
scribbles on the DSDT at run-time.
We presumed from this that Windows
must snapshot the tables at boot and run from a copy.

We decided not to copy the tables, but to use them in-place.
So we added a check for run-time corruption of the tables
and we've seen it fire only on that one Toshiba box
(which now boots with "acpi=copy_dsdt").

thanks,
Len Brown, Intel Open Source Technology Center