Re: [PATCH 3/25][V3] irq_flags / halt routines

[Chris Wright <chrisw@sous-sol.org>]

* Glauber de Oliveira Costa (gcosta@redhat.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.