Intel内存虚拟化技术分析

 

类似ASID, ASID用以区分不同进程的地址空间，切换时不用刷新TLB。VPID用来区分不同的虚拟处理器地址空间，虚拟机切换时不用刷新TLB。

 

VMM为每个VMCS分配一个唯一的VPID，在VMCS中将Enable VPID置1就可以启用该功能。

（EPT）

硬件支持的情况下，当VM ENTRY (optionally)时，EPT被激活。模式CPU同时控制两个页表，一个是Guest常规意义上的页表，该页表维护 logical page numbers (LPNs) to physical page numbers (PPNs)的映射。另一个是VMM maintains a mapping of PPNs to machine page numbers (MPNs),称为extended page tables。When EPT active, EPT base pointer (loaded on VM entry from VMCS) points to extended page tables。当VM exit事件发生时，EPT deactivated。

见

 

    因此当EPT模式address translation时，实际要遍历两个页表，注意是从guest的CR3开始地址转换。如下简图：

实际情况要复杂一些，因为guest page table()和可能是多层的，每遍历GPT的一层都可能要遍历一次EPT，所以GPT是m层，EPT是n层，遍历最大次数大约是m*n（假定TLB MISS）。下面是来自2009 虚拟化技术全国高校师资研讨班的详细示意图：

这也就是为什么huge page对VM的作用更加明显的原因，可以参看下面的链接：

Transparent Hugepage

 

EPT模式下TLB的管理相对复杂，详见INTEL Manual 25.3  CACHING TRANSLATION INFORMATION。分为三类Cache：Linear mappings（LPN->PPN）, Guest-physical mappings(PPN->MFN), Combined mappings(LPN->MFN)

后两者的内容（部分）来自EPT。可以是使用指令/INVVPID来 invalidates entries in the TLBs and paging-structure caches that were derived from extended page tables (EPT), or based on a Virtual-Processor Identifier (VPID).

 

参考资料：

不包含虚拟机相关内容，现在已经是Intel Manual的一部分，但是整合在一起看起来方便一些。

 

参考资料

Intel® 64 and IA-32 Architectures Software Developer’s Manual Volume 3B: System Programming Guide, Part 2 :chapter 25 VMX SUPPORT FOR ADDRESS TRANSLATION

 

execution Technology

 

见Intel Manual，Accesses using guest-physical addresses may cause VM exits due to EPT misconfigurations and EPT violations. An EPT misconfiguration occurs when, in the course of translation a guest-physical address, the logical processor encounters an EPT paging-structure entry that contains an unsupported value. An EPT violation occurs when there is no EPT misconfiguration but the EPT paging-structure entries disallow an access using the guest-physical address. 其中EPT violations发生的一个情况就是Translation of the guest-physical address encounters an EPT paging-structure entry that is not present，基于此可以动态创建EPT.

   此外，guest的 page faults 应该先于对应的EPT violations发生，因为EPT是根据guest的page table建立起来的。

 

4.1.2

标志位SECONDARY_EXEC_ENABLE_VPID用以控制VPID是否打开

/* VPID was disabled: now enabled. */

curr->arch.hvm_vmx.secondary_exec_control|=          ;

 

construct_vmcs有如下代码

if ( cpu_has_vmx_vpid )

        __vmwrite(VIRTUAL_PROCESSOR_ID, v->arch.hvm_vcpu.asid);

asid的分配在文件xen-4.1.2/xen/arch/x86/hvm/asid.c文件中.

 

EPT

设置见

readmsr(curr_vcpu->arch.hvm_vmx.secondary_exec_control |=

SECONDARY_EXEC_ENABLE_EPT)

 __vmwrite(EPT_POINTER, d->arch.hvm_domain.vmx.ept_control.eptp)

ept表项对应的数据结构是ept_entry_t。为ept页表建立page pool，使用的函数为hap_alloc.

 

EPT的建立完全类似demand paging机制，触发EPT Violation时建立，相应的处理函数是

=>hvm_hap_nested_page_fault,有代码如下,注释里面的PoD是populate-on-demand的意思。

 

/* Spurious fault? and log-dirty also take this path. */

    if ( p2m_is_ram(p2mt) )

    {

        /*

         * Page log dirty is always done with order 0. If this mfn resides in

         * a large page, we do not change other pages type within that large

         * page.

         */

        paging_mark_dirty(v->domain, mfn_x(mfn));

        p2m_change_type(p2m, gfn, p2m_ram_logdirty, p2m_ram_rw);

        return 1;

    }

EPT的设置在vmx_set_cr3函数中。EPT是动态创建的，利用 EPT Violation VM Exit，见tdp_page_fault函数。

ept_sync_*函数封装了invept指令。

 

VPID activated if new “enable VPID”control bit is set in VMCS

New 16-bit virtual-processor-ID field (VPID) field in VMCS
–VMM allocates unique value for each guest OS
–VMM uses VPID of 0×0000, no guest can have this VPID;

Cached linear translations are tagged with VPID value.

管理VPID资源。

Extending KVM with new Intel® Virtualization technology

http://www.mjmwired.net/kernel/Documentation/kvm/mmu.txt