linux-kernel - Re: [PATCH 3/25][V3] irq

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Date:	Wed, 15 Aug 2007 11:09:29 -0700
From:	Chris Wright <chrisw@...s-sol.org>
To:	Glauber de Oliveira Costa <gcosta@...hat.com>
Cc:	Chris Wright <chrisw@...s-sol.org>, Andi Kleen <ak@...e.de>,
	Glauber de Oliveira Costa <glommer@...il.com>,
	linux-kernel@...r.kernel.org, akpm@...ux-foundation.org,
	rusty@...tcorp.com.au, mingo@...e.hu, jeremy@...p.org,
	avi@...ranet.com, anthony@...emonkey.ws,
	virtualization@...ts.linux-foundation.org, lguest@...abs.org,
	Steven Rostedt <rostedt@...dmis.org>, kiran@...lemp.com,
	shai@...lemp.com
Subject: Re: [PATCH 3/25][V3] irq_flags / halt routines

* Glauber de Oliveira Costa (gcosta@...hat.com) wrote:
> Only caveat, is that it has to be done before smp gets in the game, and 
> with interrupts disabled. (which makes the function in vsmp.c not eligible).
> 
> My current option is to force VSMP to use PARAVIRT, as said before, and 
> then fill paravirt_arch_setup, which is currently unused, with code to 
> replace the needed paravirt_ops.fn.
> 
> I don't know if there is any method to dynamically determine (at this 
> point) that we are in a vsmp arch, and if there are not, it will have to 
> get ifdefs anyway. But at least, they are far more local.

between __cacheline_aligned_in_smp and other compile time bits based on
VSMP specific INTERNODE_CACHE, etc. I think compile time the way to go.

> I am okay with both, but after all the explanation, I don't think that 
> adding a new pvops is a bad idea. It would make things less cumbersome 
> in this case. Also, hacks like this save_fl may require changes to the 
> hypervisor, right? I don't even know where such hypervisor is, and how 
> easy it is to replace it (it may be deeply hidden in firmware)

No hypervisor change needed.  Just the pv backend needs to return 0 or
X86_EFLAGS_IF for save_flags (and similar translation on restore_flags).
Xen uses a simple shared memory flag and does something which you could
roughly translate into this:

	xen_save_flags()
	if (xen_vcpu_interrupts_enabled)
		return X86_EFLAGS_IF;
	else
		return 0;

This doesn't require any hypervisor changes.  Similarly, VSMP could do
something along the lines of:

	vsmp_save_flags()
	flags = native_save_flags();
	if (flags & X86_EFLAGS_IF) || (flags & X86_EFLAGS_AC)
		return X86_EFLAGS_IF;
	else
		return 0;

> A question raises here: Would vsmp turn paravirt_enabled to 1 ?

Probably not.  It's mostly native and I'm not sure it would want the
bits disabled from if (paravirt_enabled()) tests.
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