RE: [PATCH RFC] paravirt_ops: refactor struct paravirt_ops into smaller pv_*_ops

From: Nakajima, Jun
Date: Sat Sep 29 2007 - 12:55:23 EST

Jeremy Fitzhardinge wrote:
> Nakajima, Jun wrote:
> > Yes. For the native, "safe_halt" is "sti; hlt". The "native_halt" is
> > just "hlt". So the para_virt part of "hlt" could be moved to
> > and the "sti" part stays in pv_irq_ops.
> >
> By "sti part", you mean the full "sti; hlt" sequence of safe_halt,
> right? Since it needs to be an atomic sequence to avoid race
> conditions, so the native sequence has to be precisely "sti; hlt" to
> take advantage of the sti shadow, and other pv-backends will need
> own way to guarantee this atomicity.

To me such atomicity is provided by the "sti" instruction (i.e. the
processor begins responding to external, maskable interrupts _after_ the
next instruction is executed), and there is nothing special with that
combination "sti; hlt" (you can also have like "sti; ret", for example).
So if you define a PV ops like STI(next_instruction), "safe_halt" for
the native should be defined as STI("hlt"), and inlined as "sti; hlt".

If it's hard or we don't need to expose the semantics of "sti" other
than that, I think it's okay to have a PV operation for safe_halt.

Intel Open Source Technology Center
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