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, 10 Sep 2014 17:01:01 +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, Paolo Bonzini <pbonzini@...hat.com>
Subject: Re: [PATCH v4 4/6] kvm, mem-hotplug: Reload L1' apic access page on
 migration in vcpu_enter_guest().

On Tue, Sep 09, 2014 at 03:13:07PM +0800, tangchen wrote:
> 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 ?
> 
I acked them and CCed Paolo to this reply. I hope he will look at the series too.

--
			Gleb.
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