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SubjectRe: Intel P6 vs P7 system call performance

On Fri, 10 Jan 2003, Gabriel Paubert wrote:
>
> We cannot rely either on userspace not setting NT bit in eflags. While
> it won't cause an oops since the only instruction which ever depends on
> it, iret, has a handler (which needs to be patched, see below),
> I'm absolutely not convinced that all code paths are "NT safe" ;-)

It shouldn't matter.

NT is only tested by "iret", and if somebody sets NT in user space they
get exactly what they deserve.

> For example, set NT and then execute sysenter with garbage in %eax, the
> kernel will try to return (-ENOSYS) with iret and kill the task. As long
> as it only allows a task to kill itself, it's not a big deal. But NT is
> not cleared across task switches unless I miss something, and that looks
> very dangerous.

It _is_ cleared by task-switching these days. Or rather, it's saved and
restored, so the original NT setter will get it restored when resumed.

> I'm no Ingo, unfortunately, but you'll need at least the following patch
> (the second hunk is only a typo fix) to the iret exception recovery code,
> which used push and pops to get the smallest possible code size.

Good job.

Linus

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