linux-kernel - RE: [PATCH RFC] paravirt_ops: refactor struct paravirt_ops into smaller pv_*

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Date:	Sat, 29 Sep 2007 09:55:05 -0700
From:	"Nakajima, Jun" <jun.nakajima@...el.com>
To:	"Jeremy Fitzhardinge" <jeremy@...p.org>
Cc:	"Andi Kleen" <ak@...e.de>, "Zachary Amsden" <zach@...are.com>,
	"Rusty Russell" <rusty@...tcorp.com.au>,
	"Avi Kivity" <avi@...ranet.com>,
	"Glauber de Oliveira Costa" <glommer@...il.com>,
	"Anthony Liguori" <anthony@...emonkey.ws>,
	"Virtualization Mailing List" <virtualization@...ts.osdl.org>,
	"Ingo Molnar" <mingo@...e.hu>,
	"Thomas Gleixner" <tglx@...utronix.de>,
	"Linux Kernel Mailing List" <linux-kernel@...r.kernel.org>,
	"Alan Cox" <alan@...rguk.ukuu.org.uk>
Subject: RE: [PATCH RFC] paravirt_ops: refactor struct paravirt_ops into smaller pv_*_ops

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
pv_cpu_ops,
> > 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
their
> 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.

Jun
---
Intel Open Source Technology Center
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