Re: kernel: ldt allocation failed

["H. Peter Anvin" <hpa@zytor.com>]

Jakub Jelinek wrote:

> 
>>b) Have the kernel provide another GDT value which can be used by the
>>   single-threaded apps.
>>
> 
> Like above, a fixed address for mmap would have to be chosen, but the
> advantage would be that the TLS ABI would need no changing.
> Simply kernel would add 0x33 to GDT as 4KB -> 0xc0000000 user data segment
> and apps could put that value into %gs if not using threads.
> 
> But I think there is c) and d).
> c) is just minor modification of current ldt handling in kernel, which would
>    mean a single entry LDT (residing in task_struct) could be used instead
>    of vmalloced one - this has the disadvantage of lldt on almost every
>    context switch
> d) default to a single-entry per-cpu LDT, which only non-linux personality
>    apps and apps needing more than 1 LDT entry (threaded apps, wine/dosemu/...)
>    would not use. Non-linux apps would use current default_ldt and those
>    needing more than one LDT would use the current vmalloced mm private area.
>    If a task would be using this per-cpu LDT (common case), context switch
>    would do lldt only if previous task was not using the per-cpu LDT
>    (unlikely) and just store task_struct->thread.ldt_word_0 and ldt_word_1
>    into the per-cpu LDT (dunno how expensive is that, but IMHO it should be
>    cheaper than full load_LDT).
> 


Actually d) is probably better done by allowing CPUs to put *one* entry in
the GDT instead of requesting an LDT.  Since the LDT takes up a GDT entry
anyway, this should be simple enough.

	-hpa