Re: [PATCH 4/4] kvm, mem-hotplug: Update apic access page when it is migrated.

[Gleb Natapov]

On Fri, Jul 04, 2014 at 10:18:25AM +0800, Tang Chen wrote:
> Hi Gleb,
> 
> Thanks for the advices. Please see below.
> 
> On 07/03/2014 09:55 PM, Gleb Natapov wrote:
> ......
> >>@@ -575,6 +575,7 @@ struct kvm_arch {
> >>
> >>  	unsigned int tss_addr;
> >>  	struct page *apic_access_page;
> >>+	bool apic_access_page_migrated;
> >Better have two requests KVM_REQ_APIC_PAGE_MAP, KVM_REQ_APIC_PAGE_UNMAP IMO.
> >
> 
> vcpu->requests is an unsigned long, and we can only has 64 requests. Isn't
> adding two requests for apic page and another similar two for ept page too
> many ?  Not sure.
> 
Lets not worry about that for now. May be it is enough to have only one
KVM_REQ_APIC_PAGE_RELOAD request set apic_access_page to a new value
before sending the request and reload whatever is in apic_access_page
during KVM_REQ_APIC_PAGE_RELOAD processing. Or we can even reload
apic_access_page as part of mmu reload and reuse KVM_REQ_MMU_RELOAD.

> >>
> >>  	gpa_t wall_clock;
> >>
> >>@@ -739,6 +740,7 @@ struct kvm_x86_ops {
> >>  	void (*hwapic_isr_update)(struct kvm *kvm, int isr);
> >>  	void (*load_eoi_exitmap)(struct kvm_vcpu *vcpu, u64 *eoi_exit_bitmap);
> >>  	void (*set_virtual_x2apic_mode)(struct kvm_vcpu *vcpu, bool set);
> >>+	void (*set_apic_access_page_addr)(struct kvm *kvm, hpa_t hpa);
> >>  	void (*deliver_posted_interrupt)(struct kvm_vcpu *vcpu, int vector);
> >>  	void (*sync_pir_to_irr)(struct kvm_vcpu *vcpu);
> >>  	int (*set_tss_addr)(struct kvm *kvm, unsigned int addr);
> >>diff --git a/arch/x86/kvm/mmu.c b/arch/x86/kvm/mmu.c
> >>index c0d72f6..a655444 100644
> >>--- a/arch/x86/kvm/mmu.c
> >>+++ b/arch/x86/kvm/mmu.c
> >>@@ -3436,6 +3436,21 @@ static int tdp_page_fault(struct kvm_vcpu *vcpu, gva_t gpa, u32 error_code,
> >>  		kvm_make_request(KVM_REQ_MIGRATE_EPT, vcpu);
> >>  	}
> >>
> >>+	if (gpa == VMX_APIC_ACCESS_PAGE_ADDR&&
> >>+	    vcpu->kvm->arch.apic_access_page_migrated) {
> >Why check arch.apic_access_page_migrated here? Isn't it enough that the fault is on apic
> >address.
> >
> 
> True. It's enough. Followed.
> 
> >>+		int i;
> >>+
> >>+		vcpu->kvm->arch.apic_access_page_migrated = false;
> >>+
> >>+		/*
> >>+		 * We need update APIC_ACCESS_ADDR pointer in each VMCS of
> >>+		 * all the online vcpus.
> >>+		 */
> >>+		for (i = 0; i<  atomic_read(&vcpu->kvm->online_vcpus); i++)
> >>+			kvm_make_request(KVM_REQ_MIGRATE_APIC,
> >>+					 vcpu->kvm->vcpus[i]);
> >make_all_cpus_request(). You need to kick all vcpus from a guest mode.
> >
> 
> OK, followed. But would you please explain more about this. :)
> Why need to kick all vcpus from guest mode when making request to all vcpus
> ?
Because if you do not force other vcpus from a guest mode they will not reload
apic_access_page value till next vmexit, but since EPT page table now has a mapping
for 0xfee00000 access to this address will not cause EPT violation and will not cause
apic exit either.

> 
> >>+	}
> >>+
> >>  	spin_unlock(&vcpu->kvm->mmu_lock);
> >>
> >>  	return r;
> >>diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
> >>index c336cb3..abc152f 100644
> >>--- a/arch/x86/kvm/vmx.c
> >>+++ b/arch/x86/kvm/vmx.c
> >>@@ -3988,7 +3988,7 @@ static int alloc_apic_access_page(struct kvm *kvm)
> >>  	if (r)
> >>  		goto out;
> >>
> >>-	page = gfn_to_page(kvm, VMX_APIC_ACCESS_PAGE_ADDR>>  PAGE_SHIFT);
> >>+	page = gfn_to_page_no_pin(kvm, VMX_APIC_ACCESS_PAGE_ADDR>>  PAGE_SHIFT);
> >>  	if (is_error_page(page)) {
> >>  		r = -EFAULT;
> >>  		goto out;
> >>@@ -7075,6 +7075,12 @@ 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)
> >>+{
> >>+	if (vm_need_virtualize_apic_accesses(kvm))
> >This shouldn't even been called if apic access page is not supported. Nor
> >mmu_notifier path neither tdp_page_fault path should ever see 0xfee00000
> >address. BUG() is more appropriate here.
> >
> 
> I don't quite understand. Why calling this function here will leed to bug ?
> (Sorry, I'm not quite understand the internal of KVM. Please help.)
I didn't say that calling this function here will lead to a bug. I am saying that
if vm_need_virtualize_apic_accesses() is false this function should not be called
at all, so this check is redundant.

> 
> >
> >>+		vmcs_write64(APIC_ACCESS_ADDR, hpa);
> >>+}
> >>+
> >>  static void vmx_hwapic_isr_update(struct kvm *kvm, int isr)
> >>  {
> >>  	u16 status;
> >>@@ -8846,6 +8852,7 @@ static struct kvm_x86_ops vmx_x86_ops = {
> >>  	.enable_irq_window = enable_irq_window,
> >>  	.update_cr8_intercept = update_cr8_intercept,
> >>  	.set_virtual_x2apic_mode = vmx_set_virtual_x2apic_mode,
> >>+	.set_apic_access_page_addr = vmx_set_apic_access_page_addr,
> >svm needs that too.
> >
> 
> OK, will add one for svm.
> 
> >>  	.vm_has_apicv = vmx_vm_has_apicv,
> >>  	.load_eoi_exitmap = vmx_load_eoi_exitmap,
> >>  	.hwapic_irr_update = vmx_hwapic_irr_update,
> >>diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> >>index a26524f..14e7174 100644
> >>--- a/arch/x86/kvm/x86.c
> >>+++ b/arch/x86/kvm/x86.c
> >>@@ -5943,6 +5943,24 @@ static void vcpu_migrated_page_update_ept(struct kvm_vcpu *vcpu)
> >>  	}
> >>  }
> >>
> >>+static void vcpu_migrated_page_update_apic(struct kvm_vcpu *vcpu)
> >>+{
> >>+	struct kvm *kvm = vcpu->kvm;
> >>+
> >>+	if (kvm->arch.apic_access_page_migrated) {
> >>+		if (kvm->arch.apic_access_page)
> >>+			kvm->arch.apic_access_page = pfn_to_page(0);
> >All vcpus will access apic_access_page without locking here. May be
> >set kvm->arch.apic_access_page to zero in mmu_notifier and here call
> >  kvm_x86_ops->set_apic_access_page_addr(kvm, kvm->arch.apic_access_page);
> >
> 
> I'm a little confused. apic access page's phys_addr is stored in vmcs, and
> I think it will be used by vcpu directly to access the physical page.
> Setting kvm->arch.apic_access_page to zero will not stop it, right ?
> 
Right, kvm->arch.apic_access_page is just a shadow value for whatever is written
in vmcs. After setting it all vcpus need to update their vmcs values.

> I'm wondering what happens when apic page is migrated, but the vmcs is still
> holding its old phys_addr before the vcpu request is handled.
> 
apic page should not be migrated untill all vpus are forced out of a guest mode and
instructed to reload new value on a next guest entry. That's what we are trying to
achieve here.

--
			Gleb.
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