Freeing ACPI tables after parsing

[repost, this time with correct cc list]

AFAIK ACPI saves all information found in ACPI tables in memory somewhere else (please correct me if I'm wrong on that)

I also found this comment in e820.h:

#define E820_ACPI 3 /* usable as RAM once ACPI tables have been read */

Currently I don't see any code that would free the ACPI tables after they are read. During SLES10 testing we had some problems with the ACPI tables on some Unisys systems being in a area where the kexec kernel wanted to be loaded too. 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?

Thanks,

(quoted diff) +0 -2

-Andi
 
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Andy,
We are about to release new table code, which will not allocate memory for any tables, but just map them from their physical location, does it solve the problem you describe?

[repost, this time with correct cc list]

AFAIK ACPI saves all information found in ACPI tables in memory somewhere else (please correct me if I'm wrong on that)

I also found this comment in e820.h:

#define E820_ACPI 3 /* usable as RAM once ACPI tables have been read */

Currently I don't see any code that would free the ACPI tables after they are read.

During SLES10 testing we had some problems with the ACPI tables on some Unisys systems being in a area where the kexec kernel wanted to be loaded too.

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?

Quoted 3 lines

During SLES10 testing we had some problems with the ACPI tables on some Unisys systems being in a area where the kexec kernel wanted to be loaded too.

Why does the kexec kernel care what physical memory it is loaded into,

and how can it be guaranteed that address is not reserved by the platform firmware?

Quoted 6 lines

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?

Quoted 1 line

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.

Quoted 1 line

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.

Quoted 7 lines

Quoted 6 lines

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?

Quoted 7 lines

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.

I expect that if Windows tests that the BIOS marks the region properly and frees it, then Linux will be able to also.

Quoted 7 lines · Andi Kleen

Quoted 1 line

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.

Quoted 22 lines

Quoted 1 line

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.

Quoted 6 lines

Quoted 6 lines

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?

Quoted 7 lines

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.

Quoted 5 lines

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.