linux-kernel - Re: [PATCH v4 4/6] kvm, mem-hotplug: Reload L1' apic access page on migration in vcpu_enter

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Date:	Wed, 3 Sep 2014 18:04:49 +0300
From:	Gleb Natapov <gleb@...nel.org>
To:	tangchen <tangchen@...fujitsu.com>
Cc:	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 v4 4/6] kvm, mem-hotplug: Reload L1' apic access page on
 migration in vcpu_enter_guest().

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().

--
			Gleb.
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