Re: Better CLONE_SETTLS support for Hammer

[Ulrich Drepper]

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Andi Kleen wrote:

> You want the change, not me ;)

But I cannot test it since the kernel doesn't work.

> It should already work on the current kernel, modulo clone.
> (but arch_prctl, set_thread_area in 2.5, ldt in 2.4 etc.) 

I cannot confirm this.  I wasted a lot of time on getting it to work.
Without avail.

> It's needed for 32bit emulation at least. The code is 100% shared
> between the emulation and the native 64bit model.
> In theory it could be removed from the system call table for 64bit
> but there didn't seem a good reason to do so - after all 64bit programs
> can put their thread local data into the first 4GB and 
> fast context switches.
> 
> 
>>the use of prctl to get and set the base address.  Then internally in
>>the prctl call map it to either the use of a 32 base address segment or
>>use the MSR.
> 
> 
> The problem is that the 64bit base has different semantics. 
> 
> When you use a segment register you have to do:
> 
> 	call kernel to set gdt/ldt
> 	movl index,%%fs
> 
> But when the kernel did set the 64bit base in the kernel call the
> following movl to the selector would destroy it again
> 
> Loading the index inside the system call would also be problematic:
> First it would be different from what i386 does, causing porting headaches.
> Also you could not easily do it from a different thread unlike the 
> LDT load.
> 	
> 
> 
>>This way whoever needs a segment base address can preferably allocate it
>>in the low 4GB, but if it fails the kernel support still work.  And with
>>the same interface.  Currently this is not the case and this is not
>>acceptable.
> 
> 
> That should already work and it is in fact how I imagined this to be: 
> 
> do MAP_32BIT - if yes use set_thread_area or an LDT entry;
> 
> if not use arch_prctl
> 
> The NPTL signal race problem for the clones in case you have a 64bit
> base is a bit ugly though I agree.
> 
> I don't like your patch currently because it'll guarantee slow 
> context switch times for 64bit.
> 
> Automatic switching based on the set bits in the base may be possible 
> (in fact I had something like this in set_thread_area for some time, but 
> removed it because of the ugly semantics because set_thread_area doesn't
> already load the selector). If the selector load is forced
> in clone however it would not be as weird, just only somewhat
> ugly. You'll have to guarantee in user space then that you don't
> reload it.
> 
> Real solution would be Windowish - Create clone7() with both
> selectors and bases 
> 
> [I suspect 2.8 and 3.0 will get that anyways as experience
> on other operating systems who started on the same path 
> shows. e.g. AmigaOS grew more CreateTask
> variants with more arguments with each release until they eventually
> settled on passing tag lists.]
> 
> -Andi
> 

- -- 
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Ulrich Drepper \    ,-----------------'   \ Mountain View, CA 94041 USA
Red Hat         `--' drepper at redhat.com `---------------------------
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