Re: [PATCH v4 4/6] kvm, mem-hotplug: Reload L1' apic access page on migration in vcpu_enter_guest().

[tangchen]

Hi Gleb,

On 09/03/2014 11:04 PM, Gleb Natapov wrote:
> On Wed, Sep 03, 2014 at 09:42:30AM +0800, tangchen wrote:
> > Hi Gleb,
> >
> > On 09/03/2014 12:00 AM, Gleb Natapov wrote:
> > > ......
> > > +static void vcpu_reload_apic_access_page(struct kvm_vcpu *vcpu)
> > > +{
> > > +	/*
> > > +	 * apic access page could be migrated. When the page is being migrated,
> > > +	 * GUP will wait till the migrate entry is replaced with the new pte
> > > +	 * entry pointing to the new page.
> > > +	 */
> > > +	vcpu->kvm->arch.apic_access_page = gfn_to_page(vcpu->kvm,
> > > +				APIC_DEFAULT_PHYS_BASE >> PAGE_SHIFT);
> > > +	kvm_x86_ops->set_apic_access_page_addr(vcpu->kvm,
> > > +				page_to_phys(vcpu->kvm->arch.apic_access_page));
> > > I am a little bit worried that here all vcpus write to vcpu->kvm->arch.apic_access_page
> > > without any locking. It is probably benign since pointer write is atomic on x86. Paolo?
> > >
> > > Do we even need apic_access_page? Why not call
> > >   gfn_to_page(APIC_DEFAULT_PHYS_BASE >> PAGE_SHIFT)
> > >   put_page()
> > > on rare occasions we need to know its address?
> > Isn't it a necessary item defined in hardware spec ?
> vcpu->kvm->arch.apic_access_page? No. This is internal kvm data structure.
>
> > I didn't read intel spec deeply, but according to the code, the page's
> > address is
> > written into vmcs. And it made me think that we cannnot remove it.
> We cannot remove writing of apic page address into vmcs, but this is not done by
> assigning to vcpu->kvm->arch.apic_access_page, but by vmwrite in set_apic_access_page_addr().

OK, I'll try to remove kvm->arch.apic_access_page and send a patch for 
it soon.

BTW, if you don't have objection to the first two patches, would you 
please help to
commit them first ?

Thanks.

> --
> 			Gleb.
> .

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