RE: [Xen-devel] Re: Reproducable data corruption on xen-unstable

[Robin Green <greenrd@xxxxxxxxxxxxx>]

On Sat, 5 Feb 2005, Robin Green wrote:
> On the assumption that this _is_ an FP save/restore bug,
Update: I have narrowed down this bug

I have confirmed that there IS definitely an FP save/restore bug 
with this kernel/xen combination (i.e. I've eliminated the possibility 
that it was just a non-floating-point-related bug)! I identified it using 
a different test case (running wget -d in a konsole), and I have established
that it is case 1 in the list of possible causes I gave, namely:

> 1. Something leaves the FPU in a state where it has bogus data in it,
>    but it won't trap to tell the kernel to restore the old, correct data
More specifically, in this particular case, according to my printf's, what 
happened was:
A syscall was made (connect). Immediately before the syscall, the 
floating-point stack was empty; immediately after the syscall, the 
floating-point stack was nonempty, and the TS flag (Task Switch) was _cleared_.
(Source code and output available on request.)

This may not immediately cause problems. But over time, it would tend to 
lead to floating-point stack overflow, which leads to floating-point 
calculations generating bogus output.
So, in theory there are two possible algorithms which the kernel could be 
supposed to be following to avoid this situation.
A. Always set TS on task switch (Seems like the logical choice!)

B. Always set TS on task switch - except when the FPU has not been used
by the switched-to process, in which case do an FINIT on task switch. 
(This seems pointlessly complicated and slow, so I doubt the kernel 
follows this approach.)
So, it looks like we are looking for a code path in which TS doesn't end
up set after a task switch. (And it might be specifically to do with
syscalls.)

I will look for one - but does anyone have any ideas for what that code 
path might be, or how I could efficiently debug the kernel (while in X, 
remember, because this doesn't seem to occur in text mode!) to find out 
what that code path is? I don't have a serial console.
--
Robin


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