Re: Freeing ACPI tables after parsing

[Alexey Starikovskiy <alexey_y_starikovskiy@linux.intel.com>]

Len Brown wrote:
> On Monday 21 August 2006 18:06, Andi Kleen wrote:
> 
>>> Why does the kexec kernel care what physical memory it is loaded into,
>> At least the one in SLES10 was linked to a fixed address (64MB@64MB)
>> In the future it will be relocatable, but still there will be defaults
>> passed in advance.
>>
>> Unisys ended up fixing it with a new BIOS, but I can well see this
>> happening again.
> 
> Okay, good to know this is not urgent.  I think we should proceed with our
> 1-copy-of-the-tables change, and we can independently change it later to
> copy them and reclaim the original space if that turns out to work.
>  
>>> and how can it be guaranteed that address is not reserved by the platform firmware?
>> I don't think there is a fool proof method right now, just heuristics
>> However freeing ACPI would avoid some of the potential problems.
>>
>>>> Also on other systems the ACPI tables are not 
>>>> exactly at the end but in the middle of the memory map and for some applications
>>>> it might be better to have a lot of physical continuous memory.
>>>>
>>>> So are there any plans to free the BIOS supplied ACPI tables after parsing
>>>> or is there some obstacle to that that I'm missing?
>>> As Alexey wrote, he and Bob have already deleted the kernel's new copy
>>> of the tables in favor of using them 'in place'.
>>>
>>> I guess I'm not clear on the requirement here.
>>> If we were to alter that plan to make a kernel copy -- and release the original BIOS memory --
>>> assuming it is marked such that we can release it -- how do we know that
>>> we are not dropping the new copy someplace that kexec doesn't want it?
>> There are other allocations anyways and the allocators tend to cluster, so one would
>> assume it ends up being ok. If there are conflicts they can be handled in
>> one single place in Linux. But Linux can't affect the BIOS' choice of table
>> placement.
>>
>> Also I suspect the kernel can store it more efficiently in memory. When the tables
>> are reserved in memory they will be always rounded up/down to page boundaries,
>> while a kernel allocation can fit it better. At least for some of them (like fadt)
>> the kernel seems to use globals already too, so the original mapping wouldn't be 
>> needed.
> 
> This  trade-off may not be risk-free.
> I expect that if Windows tests that the BIOS marks the region properly
> and frees it, then Linux will be able to also.  Otherwise Linux will fight
> a BIOS battle it can't win w/o Windows acting as a BIOS validation test for us.

Actually FACS table has a global_lock in it (which the BIOS writes too), so it is not movable,
and as all tables are laid contiguously in physical memory, moving some and
leaving FACS is not an option.

Regards,
	Alex.