Re: [uml-devel] [Patch] Save FPU registers between task switches

[Ingo van Lil <inguin@gmx.de>]

Jeff Dike wrote:

>> some time ago Jeff prepared a patch [1] for UML to stop saving the
>> process FP state between task switches. The assumption was that since
>> with SKAS0 every guest process runs inside a host process context the
>> host OS will take care of keeping the proper FP state. Unfortunately
>> this is not true for multi-threaded applications, where all guest
>> threads share a single host process context yet all may use the FPU on
>> their own. Although I haven't verified it I suspect things to be even
>> worse in SKAS3 mode where all guest processes run inside a single host
>> process.
> 
> This seems plausible.  Does reverting the patch fix any process crashes?

Hi Jeff,

the tiny test program I posted a few weeks ago [1] reproducibly crashes 
after a few seconds without the patch. Additionally I've been running 
UML with the patch for about a week now and I haven't seen the 
occasional erratic results that lead me to investigate the issue since then.

I've never been using the SKAS3 patch, so I cannot comment on that. I 
had hoped that Stanislav Meduna could try whether the patch fixes the 
problems he reported last September [2].

Regards,
Ingo

[1] http://marc.info/?l=user-mode-linux-devel&m=123912656515459&w=2
[2] http://marc.info/?l=user-mode-linux-devel&m=121819897821728&w=2

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