Re: [PATCH v2 5/5] kvm, mem-hotplug: Do not pin apic access page in memory.

[Tang Chen]

On 07/15/2014 08:40 PM, Gleb Natapov wrote:
> On Tue, Jul 15, 2014 at 08:28:22PM +0800, Tang Chen wrote:
> > On 07/15/2014 08:09 PM, Gleb Natapov wrote:
> > > On Tue, Jul 15, 2014 at 01:52:40PM +0200, Jan Kiszka wrote:
> > ......
> > > > I cannot follow your concerns yet. Specifically, how should
> > > > APIC_ACCESS_ADDR (the VMCS field, right?) change while L2 is running? We
> > > > currently pin/unpin on L1->L2/L2->L1, respectively. Or what do you mean?
> > > I am talking about this case:
> > >           if (cpu_has_secondary_exec_ctrls()) {a
> > >           } else {
> > >               exec_control |=
> > >                  SECONDARY_EXEC_VIRTUALIZE_APIC_ACCESSES;
> > >              vmcs_write64(APIC_ACCESS_ADDR,
> > >                  page_to_phys(vcpu->kvm->arch.apic_access_page));
> > >           }
> > > We do not pin here.
> >
> > Hi Gleb,
> >
> >
> > 7905                 if (exec_control&
> > SECONDARY_EXEC_VIRTUALIZE_APIC_ACCESSES) {
> > ......
> > 7912                         if (vmx->nested.apic_access_page) /* shouldn't
> > happen */
> > 7913 nested_release_page(vmx->nested.apic_access_page);
> > 7914                         vmx->nested.apic_access_page =
> > 7915                                 nested_get_page(vcpu,
> > vmcs12->apic_access_addr);
> >
> > I thought you were talking about the problem here. We pin
> > vmcs12->apic_access_addr
> > in memory. And I think we should do the same thing to this page as to L1 vm.
> > Right ?
> Nested kvm pins a lot of pages, it will probably be not easy to handle all of them,
> so for now I am concerned with non nested case only (but nested should continue to
> work obviously, just pin pages like it does now).

True. I will work on it.

And also, when using PCI passthrough, kvm_pin_pages() also pins some 
pages. This is
also in my todo list.

But sorry, a little strange. I didn't find where 
vmcs12->apic_access_addr is allocated
or initialized... Would you please tell me ?

> > ......
> > 7922                         if (!vmx->nested.apic_access_page)
> > 7923                                 exec_control&=
> > 7924 ~SECONDARY_EXEC_VIRTUALIZE_APIC_ACCESSES;
> > 7925                         else
> > 7926                                 vmcs_write64(APIC_ACCESS_ADDR,
> > 7927 page_to_phys(vmx->nested.apic_access_page));
> > 7928                 } else if
> > (vm_need_virtualize_apic_accesses(vmx->vcpu.kvm)) {
> > 7929                         exec_control |=
> > 7930 SECONDARY_EXEC_VIRTUALIZE_APIC_ACCESSES;
> > 7931                         vmcs_write64(APIC_ACCESS_ADDR,
> > 7932 page_to_phys(vcpu->kvm->arch.apic_access_page));
> > 7933                 }
> >
> > And yes, we have the problem you said here. We can migrate the page while L2
> > vm is running.
> > So I think we should enforce L2 vm to exit to L1. Right ?
> We can request APIC_ACCESS_ADDR reload during L2->L1 vmexit emulation, so
> if APIC_ACCESS_ADDR changes while L2 is running it will be reloaded for L1 too.

apic pages for L2 and L1 are not the same page, right ?

I think, just like we are doing in patch 5/5, we cannot wait for the 
next L2->L1 vmexit.
We should enforce a L2->L1 vmexit in mmu_notifier, just like 
make_all_cpus_request() does.

Am I right ?

Thanks.

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