Xen Memory Management

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Below was cited fromhttps://wiki.cs.dartmouth.edu/nihal/doku.php/xen:memory. Changes subject to this site.

All low-level memory operations go through Xen.
Guest OSes are responsible for allocating and initializing PTs for processes (restricted to read only access)
- allocates and initialize a page and register it with Xen to serve as the new PT
Direct page writes are intercepted, validated and applied by the Xen VMM
- update can be batched into a single hypercall (reduce cost of entering/exiting Xen)
page_info struct associated with each machine page frame
- page type (none, l1, l2, l3, l4, LDT, GDT, RW)
- reference count – number of references to the page
- page frame can be reused only when unpinned and its reference count is zero
Each domain has a maximum and current memory allocation
- max allocation is set at domain creation time and cannot be modified
PT updates
- hypercall –> mmu_update()
- writable page tables –> vm_assist()

Xen exists in the top 64MB (0xFC000000 – 0xFFFFFFFF) section of every guest virtual address space (TLB flush avoided when entering/leaving the hypervisor)
- not accessible or remappable by guest OSes.
“fast handler” for system calls - direct access from app into guestOS, without going through Xen
- muse execute outside Ring 0
Each guest supports a “ballon” memory management driver - that is used by the VMM to dynamically adjust the guest’s memory usage
Page fault handling
- faulting address is written into an extended stack frame on the guestOSstack (normally the faulting address is read from a privileged processor register (CR2))
In terms of page protection, Ring1/2 are considered to be part of ‘supervisor mode’. The WP bit in CR0 controls whether read-only restrictions are respected in supervisor mode – if the bit is clear then any mapped page is writable. Xen gets around this by always setting the WP bit and disallowing updates to it. xen/arch/x86/boot/x86_32.S#153
Xen provides a domain with a list of machine frames during bootstrapping, and it is the domain’s responsibility to create the pseudo-physical address space from this

No guarantee that a domain will receive a contiguous stretch of physical memory. Most OSes do not have good support for operating in a fragmented physical address space.

Machine memory
- entire amount of memory installed in the machine (physical memory)
- 4kB machine page frames numbered consecutively starting from 0.

Pseudo-physical memory
- per-domain abstraction.
- allows a guestOSto consider its memory allocation to consist of a contiguous range of physical page frames starting at physical frame 0.

machine-to-physical table
- globally readable table maintained by Xen
- records the mapping from machine addresses to pseudo-physical addresses
- table size is proportional to the amount of RAM installed in the machine

physical-to-machine table
- per-domain table which performs the inverse (physical-to-machine) mapping.
- table size is proportional to the memory allocation of the given domain.

(XEN) VIRTUAL MEMORY ARRANGEMENT (for DOM0)
(XEN) Loaded kernel: c0100000→c042e254
(XEN) Init. ramdisk: c042f000→c07fca00
(XEN) Phys-Mach map: c07fd000→c086e894 == 454MB(as can be verified by: xm list)
(XEN) Start info: c086f000→c0870000
(XEN) Page tables: c0870000→c0874000 == 16MB
(XEN) Boot stack: c0874000→c0875000
(XEN) TOTAL: c0000000→c0c00000
(XEN) ENTRY ADDRESS: c0100000

x86-32 Xen supports only guests with 2-level page tables. PGD = l2, PTE =l1

How to intercept interrupts from guest domains
http://lists.xensource.com/archives/html/xen-devel/2006-09/msg00597.html
http://lists.xensource.com/archives/html/xen-devel/2006-09/msg00604.html

Page fault handling for Xen guests
http://lists.xensource.com/archives/html/xen-devel/2006-02/msg00263.html

show pagetable walk if guest cannot handle page
http://lists.xensource.com/archives/html/xen-devel/2006-09/msg00612.html

Memory management, mapping, paging questions...
http://lists.xensource.com/archives/html/xen-devel/2006-10/msg01151.html

How to intercept memory operation in Xen
http://lists.xensource.com/archives/html/xen-devel/2006-11/msg00659.html
http://lists.xensource.com/archives/html/xen-devel/2006-11/msg00664.html
http://lists.xensource.com/archives/html/xen-devel/2006-11/msg00717.html

alert message from dom0 to domU
http://lists.xensource.com/archives/html/xen-devel/2006-12/msg00967.html

Share Memory Between DomainU and Domain0
http://lists.xensource.com/archives/html/xen-devel/2006-12/msg01008.html

Call hypercall straightly from user space
http://lists.xensource.com/archives/html/xen-devel/2006-12/msg01061.html

xen/arch/x86/traps.c#do_page_fault –> fixup_page_fault –> mm.c#ptwr_do_page_fault

xen-3.0.2-2/xen/arch/x86/setup.c#__start_xen()
                |                                 \
                v                                  \
xen-3.0.2-2/xen/common/domain.c#domain_create()     \
                |                                    \
                v                                     \
xen-3.0.2-2/xen/arch/x86/domain.c#arch_domain_create() \
                                                        \
                                                         v
                xen-3.0.2-2/xen/arch/x86/domain_build.c#construct_dom0()

Xen-ELF image vmlinux-syms-2.6.16-xen has a special'__xen_guest' section


Xen hypercall table:
/xen-3.0.2-2/xen/arch/x86/x86_32/entry.S


#I think this is called when DOM0 attempts to create a DOMU
xen-3.0.2-2/xen/common/dom0_ops.c#do_dom0_op()




trousers-0.2.7/src/tspi/spi_tpm.c#Tspi_TPM_Quote()
                |
                v
trousers-0.2.7/src/tcsd_api/calltcsapi.c#TCSP_Quote()
                |
                v
trousers-0.2.7/src/tcsd_api/tcstp.c#TCSP_Quote_TP()
                |
                v
trousers-0.2.7/src/tcsd_api/tcstp.c#sendTCSDPacket()

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