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Date: Tue, 8 Jun 2010 17:25:52 +0100
From: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
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To: Konrad Rzeszutek Wilk <konrad.wilk@Oracle.com>
CC: Stefano Stabellini <Stefano.Stabellini@eu.citrix.com>,
       "jeremy@goop.org" <jeremy@goop.org>,
       "xen-devel@lists.xensource.com" <xen-devel@lists.xensource.com>,
       "Yaozu (Eddie) Dong" <eddie.dong@intel.com>,
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       "ddutile@redhat.com" <ddutile@redhat.com>,
       "sheng@linux.intel.com" <sheng@linux.intel.com>
Subject: Re: [Xen-devel] [PATCH 02/12] early PV on HVM
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On Tue, 8 Jun 2010, Konrad Rzeszutek Wilk wrote:
> On Tue, Jun 08, 2010 at 04:55:33PM +0100, Stefano Stabellini wrote:
> > On Tue, 8 Jun 2010, Konrad Rzeszutek Wilk wrote:
> > > > > > +	HYPERVISOR_shared_info = (struct shared_info *)shared_info_page;
> > > > > > +
> > > > > > +	/* Don't do the full vcpu_info placement stuff until we have a
> > > > > > +	   possible map and a non-dummy shared_info. */
> > > > > 
> > > > > Might want to mention where the full vpcu placement is done.
> > > > 
> > > > The comment is not accurate, we actually don't do any vcpu_info
> > > > placement on hvm because it is not very useful there.
> > > > Better just to remove the comment (I have done so in my tree).
> > > > 
> > > > > > +	per_cpu(xen_vcpu, 0) = &HYPERVISOR_shared_info->vcpu_info[0];
> > > > > 
> > > So.. what is the purpose of the per_cpu(xen_vcpu, 0) then?
> > > 
> > 
> > the vcpu info placement memory area is stored in per_cpu(xen_vcpu_info, cpu);
> > per_cpu(xen_vcpu, cpu) is just a pointer to that area if it is
> > available, otherwise it points to the vcpu_info struct in the shared
> > info page.
> 
> I was just wondering why are we doing this when you say:
> " don't do any vcpu_info placement on hvm because it is not very useful there."
> 
> So if it is not useful, why do it?
> 

I think Jeremy replied to your question better than me: we still need
the vcpu_info stuff for the timer and event channels, but we don't need
it to be at a specific address in kernel memory.
That is useful only for the following pvops operations:

pv_irq_ops.save_fl = __PV_IS_CALLEE_SAVE(xen_save_fl_direct);
pv_irq_ops.restore_fl = __PV_IS_CALLEE_SAVE(xen_restore_fl_direct);
pv_irq_ops.irq_disable = __PV_IS_CALLEE_SAVE(xen_irq_disable_direct);
pv_irq_ops.irq_enable = __PV_IS_CALLEE_SAVE(xen_irq_enable_direct);
pv_mmu_ops.read_cr2 = xen_read_cr2_direct;

none of which are used in the hvm case.