linux-kernel - Re: [PATCH v5 4/7] kvm, mem-hotplug: Reload L1' apic access page on migration in vcpu_enter

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Date:	Thu, 11 Sep 2014 12:47:16 +0200
From:	Paolo Bonzini <pbonzini@...hat.com>
To:	Gleb Natapov <gleb@...nel.org>
CC:	Tang Chen <tangchen@...fujitsu.com>, mtosatti@...hat.com,
	nadav.amit@...il.com, jan.kiszka@....de, kvm@...r.kernel.org,
	laijs@...fujitsu.com, isimatu.yasuaki@...fujitsu.com,
	guz.fnst@...fujitsu.com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v5 4/7] kvm, mem-hotplug: Reload L1' apic access page
 on migration in vcpu_enter_guest().

Il 11/09/2014 12:12, Gleb Natapov ha scritto:
> On Thu, Sep 11, 2014 at 11:21:49AM +0200, Paolo Bonzini wrote:
>> Il 11/09/2014 07:38, Tang Chen ha scritto:
>>> diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
>>> index 63c4c3e..da6d55d 100644
>>> --- a/arch/x86/kvm/vmx.c
>>> +++ b/arch/x86/kvm/vmx.c
>>> @@ -7093,6 +7093,11 @@ static void vmx_set_virtual_x2apic_mode(struct kvm_vcpu *vcpu, bool set)
>>>  	vmx_set_msr_bitmap(vcpu);
>>>  }
>>>  
>>> +static void vmx_set_apic_access_page_addr(struct kvm *kvm, hpa_t hpa)
>>> +{
>>> +	vmcs_write64(APIC_ACCESS_ADDR, hpa);
>>
>> This has to be guarded by "if (!is_guest_mode(vcpu))".
>>
> We do need to write it if L1 and L2 share APIC_ACCESS_ADDR and skip
> it otherwise, no?

Yes, but this would be handled by patch 6:

 		} else if (vm_need_virtualize_apic_accesses(vmx->vcpu.kvm)) {
+			struct page *page = gfn_to_page(vmx->vcpu.kvm,
+				APIC_DEFAULT_PHYS_BASE >> PAGE_SHIFT);
 			exec_control |=
 				SECONDARY_EXEC_VIRTUALIZE_APIC_ACCESSES;
-			vmcs_write64(APIC_ACCESS_ADDR,
-				page_to_phys(vcpu->kvm->arch.apic_access_page));
+			vmcs_write64(APIC_ACCESS_ADDR, page_to_phys(page));
+			/*
+			 * Do not pin apic access page in memory so that memory
+			 * hotplug process is able to migrate it.
+			 */
+			put_page(page);
 		}

However, this is also useless code duplication because the above snippet could
reuse vcpu_reload_apic_access_page too.

So I think you cannot do the is_guest_mode check in
kvm_vcpu_reload_apic_access_page and also not in
vmx_reload_apic_access_page.  But you could do something like

kvm_vcpu_reload_apic_access_page(...)
{
	...
	kvm_x86_ops->reload_apic_access_page(...);
}
EXPORT_SYMBOL_GPL(kvm_vcpu_reload_apic_access_page);

/* used in vcpu_enter_guest only */
vcpu_reload_apic_access_page(...)
{
	if (!is_guest_mode(vcpu))
		kvm_vcpu_reload_apic_access_page(...)
}

Paolo
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