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Introduction of AMDAdvanced Virtual InterruptControllerXenSummit 2012Wei HuangAugust 2012

What is AVIC? AVIC is Advanced Virtual Interrupt Controller A virtual APIC to guest OSs with hardware acceleration Enhancement to the AMD SVM architecture Changes to CPU, NorthBridge, and IOMMU2 | Introduction of AMD AVIC | Xen Summit | August, 2012

Agenda Motivation for AVIC AVIC Architecture – APIC backing page – Physical & logical APIC ID tables – Door bell & IOMMU extension Hypervisor Design for AVIC Summary3 | Introduction of AMD AVIC | Xen Summit | August, 2012

Local APIC in x86 Local APIC (LAPIC) is vital for x86 SMP system Handles various interrupt sources: – Inter-processor interrupts (IPIs) – I/O devices – APIC timer-generated interrupts – Internal events CPU 1 CPU 2 CPU 3 CPU 4 Local APIC Local APIC Local APIC Local APIC Interrupt Interrupt Interrupt Interrupt IPIs IPIs IPIs IPIs messages messages messages messages Interrupt messages External I/O APIC interrupts5 | Introduction of AMD AVIC | Xen Summit | August, 2012

Many LAPIC Registers… *AMD APM Vol 26 | Introduction of AMD AVIC | Xen Summit | August, 2012

LAPIC + Virtualization = Slow1. APIC register reads/writes 2. Inter-processor Interrupts Guest OS Guest OS vCPU vCPU X vCPU X APIC Reg Rd/Wrt IPI Hypervisor Hypervisor3. I/O interrupts from peripherals (e.g., pass-through device) Guest OS device vCPU mem Hypervisor IOMMU DMA I/O dev interrupt Device7 | Introduction of AMD AVIC | Xen Summit | August, 2012

Attacking Problems with AVIC We define new architectural components to present a virtualized APIC to guests, thus allowing most APIC accesses and interrupt delivery into the guests directly. AVIC components: Component Problem Solved vAPIC Direct access to APIC registers Physical & logical APIC ID tables Mapping of physical & virtual cores (scheduling) Doorbell interrupt Interrupt delivery IOMMU extension I/O interrupt delivery8 | Introduction of AMD AVIC | Xen Summit | August, 2012

Agenda Motivation for AVIC AVIC Architecture – APIC backing page – Physical & logical APIC ID tables – Doorbell & IOMMU extension Hypervisor Design for AVIC Summary9 | Introduction of AMD AVIC | Xen Summit | August, 2012

vAPIC vAPIC is a virtual APIC to the guest – Backed by a 4KB memory page – Allocated and initialized by hypervisor – vAPIC pages are allocated on a per-vCPU basis – Contains storage for guest APIC values for the vCPU – Mapped into the guest physical address space APIC Backing Page ICR guest memory ICR ICR IRR IRR IRR ISR ISR ISR guest APIC ICR TPR ICR TPR ICR TPR IRR EOI IRR EOI IRR EOI BACKING_PAGE ptr ISR myAPIC ISR myAPIC ISR Physical APIC Table Physical APIC Table TPR Logical APIC Table TPR Logical APIC Table TPR V_APIC_BAR EOI EOI = EOI VMCB10 | Introduction of AMD AVIC | Xen Summit | August, 2012

Handling Guest APIC Accesses Four types of actions for guest APIC accesses: Action VMEXIT? Details ALLOWED NO The field will be updated (for Write) or read (for Read). ACCLERATED NO The field will be updated and CPU will take further actions, such as sending IPI to CPUs. TRAP YES The field will be updated and CPU will take VMEXIT immediately after access. FAULT YES The field will NOT be updated and CPU will take VMEXIT immediately before the access. Example Offset APIC Register Name Read Write 200-270h Interrupt Request Register (IRR) ALLOWED FAULT 300h Interrupt Command Register Low (ICR) ALLOWED ACCLERATED or TRAP11 | Introduction of AMD AVIC | Xen Summit | August, 2012

Regarding APIC Acceleration… Three performance-critical areas are accelerated – TPR reads and writes – EOI writes – ICRL writes12 | Introduction of AMD AVIC | Xen Summit | August, 2012

How About vCPU Scheduling? Hypervisor schedules guest VM’s VCPUs on-the-fly – VCPUs can be ON or OFF – VCPUs can be migrated AVIC uses physical and logical APIC ID tables to show VCPU’s mapping relationship with physical cores14 | Introduction of AMD AVIC | Xen Summit | August, 2012

Physical APIC ID Table Purpose: Used by the hardware to route the virtual interrupt messages to the proper physical core vAPIC Backing Page Guest CPU 4 Physical APIC Table Guest APIC ID 5 entry VMCB Guest APIC ID 4 entry Guest APIC ID 3 Doorbell Destination ID entry AVIC_PHYSICAL_TABLE ptr15 | Introduction of AMD AVIC | Xen Summit | August, 2012

Logical APIC ID Table Purpose: – Maps guest logical APIC IDs to guest physical APIC IDs – Used to route logically addressed IPIs within a guest VM vAPIC Backing Page Guest CPU 0 Physical APIC Table Logical APIC Table Guest APIC ID 2 entry Guest logical 7 Guest APIC ID 1VMCB to physical ID entry Guest logical 6 Guest APIC ID 0 to physical 0 entry Guest logical 5 Doorbell Destination ID to physical IDAVIC_PHYSICAL_TABLE ptr16 | Introduction of AMD AVIC | Xen Summit | August, 2012

Agenda Motivation for AVIC AVIC Architecture – APIC backing page – Physical & logical APIC ID tables – Doorbell & IOMMU extension Hypervisor Design for AVIC Summary17 | Introduction of AMD AVIC | Xen Summit | August, 2012

Doorbell A new interrupt mechanism that is used to deliver guest interrupts to a specific physical core Doorbell interrupts are sent in three ways: Doorbell Source Reason AVIC hardware In response to a guest ICRL write for a supported IPI type IOMMU An incoming I/O interrupt is remapped to an AVIC mode guest System software Writing to the doorbell interrupt MSR18 | Introduction of AMD AVIC | Xen Summit | August, 2012

IOMMU Extension The AVIC architecture leverages the existing IOMMU interrupt redirection mechanism to provide a new guest-delivered interrupt type A new field in the IOMMU device table specifies whether AVIC is available IOMMU uses doorbell mechanism to delivery interrupts into guest VMs19 | Introduction of AMD AVIC | Xen Summit | August, 2012

How to Design a Hypervisor for AVIC? At guest start-up: – Allocate and initialize one vAPIC page for each vCPU – Allocate one physical APIC table and logical APIC table for the guest – Set up VMCB pointers for these structures and enable AVIC mode in VMCB – Set up IOMMU tables for direct-assigned I/O devices While running: – Handle un-accelerated cases similar to current approach – Update physical APIC table and IOMMU IRTE entries to change running status whenever a vCPU is moved to or from a physical CPU21 | Introduction of AMD AVIC | Xen Summit | August, 2012

Setting up AVIC TablesvAPIC backing pages and physical/logical ID tables are allocated in hostphysical memoryRelated VMCB fields: VMCB Field Details V_APIC_BAR Guest physical base address of the virtual APIC. AVIC_BACKING_PAGE Host physical address of the APIC backing page for the associated VM. AVIC_LOGICAL_PAGE Host physical address of the logical APIC look-up table for the associated VM. AVIC_PHYSICAL_PAGE Host physical address for the physical APIC look-up table for the associated VM.22 | Introduction of AMD AVIC | Xen Summit | August, 2012

Handle New VMEXITs VMEXIT 401h – For incomplete IPI Delivery – This VMEXIT describes the specific reason for the IPI delivery failure VMEXIT 402h – For ccess to un-accelerated vAPIC field – This VMEXIT indicates the offset of the un-accelerated vAPIC register, as well as whether a read or write operation was attempted23 | Introduction of AMD AVIC | Xen Summit | August, 2012

Summary AVIC is an hardware extension to AMD SVM for APIC acceleration AVIC targets critical APIC operations for optimal performance – APIC read/write accesses – Interrupt delivery The design of AVIC is simple for fast hypervisor integration We expect AVIC to eliminate lots of overhead introduced by virtual local APIC24 | Introduction of AMD AVIC | Xen Summit | August, 2012

Questions?25 | Introduction of AMD AVIC | Xen Summit | August, 2012

Trademark AttributionAMD, the AMD Arrow logo and combinations thereof are trademarks of Advanced Micro Devices, Inc. in the United Statesand/or other jurisdictions. Other names used in this presentation are for identification purposes only and may be trademarks oftheir respective owners.©2012 Advanced Micro Devices, Inc. All rights reserved.26 | Introduction of AMD AVIC | Xen Summit | August, 2012

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Introduction of AMD Virtual Interrupt Controller