Re: [PATCH v10 00/18] Enable FSGSBASE instructions

[Andi Kleen]

> So this is a check that checks if you're running in user mode if
> you have a debug trap with single step, but somehow it triggered
> for a user segment.
> 
> Probably the regs got corrupted. 
> 
> Sasha, I suspect you're missing a mov %rsp,%rdi somewhere in the
> debug entry path that sets up the regs argument for the C code.

... Ah never mind. Thomas has a better explanation.

-Andi