Xen Memory Management
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 guest OS, 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 guest OSstack (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 guest OSto 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
Information related to shadowing
http://lists.xensource.com/archives/html/xen-devel/2006-11/msg00319.html
http://lists.xensource.com/archives/html/xen-devel/2006-11/msg00793.html
http://lists.xensource.com/archives/html/xen-devel/2006-11/msg00802.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()