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SubjectRe: Sysenter crash with Nested Task Bit set
Linus Torvalds wrote:
> Final note (I promise): now that we save/restore eflags again, we
> should probably revert the set_iopl_mask() in task switching too. However,
> that apparently has some para-virtualization issues, so I'm going to
> ignore that part from now.

I'm wondering if we shouldn't have a (__)switch_to paravirt hook, so we
can wrap the context switch in whatever we like.

> However, I'd really like people who know and care about the
> paravirtualization to take a good long look at it: because right now, with
> the addition of the eflags save/restore, the set_iopl_mask() in
> __switch_to() is entirely useless for non-virtualized environments, afaik.

Hm. Zach removed the pushf/popf in switch_to this last Sept, with the
comment "The pushf/popf in switch_to are ONLY used to switch IOPL.
Making this explicit in C code is more clear. This pushf/popf pair was
added as a bugfix for leaking IOPL to unprivileged processes when using
sysenter/sysexit based system calls (sysexit does not restore flags)."

> Zack added to the cc. Who else needs to know?
Rusty, Chris Wright and me.

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