[Xen-devel] [DOCS/PATCH] http://wiki.xen.org/wiki/X86_Paravirtualised_Memory_Management changes

[Konrad Rzeszutek Wilk <konrad@xxxxxxxxxx>]

These changes would allow the above mention Wiki to reference to
more materials. This is what I had in mind to add to the Wiki:

diff --git a/intro.txt b/intro.txt
index b902b1e..72d3ca1 100644
--- a/intro.txt
+++ b/intro.txt
@@ -339,6 +339,188 @@ into the page table base register but will not be 
explicitly pinned.
 The initial virtual and pseudo-physical layout of a new guest is
 described 
[http://xenbits.xen.org/docs/unstable/hypercall/include,public,xen.h.html#incontents_startofday
 here].
 
+
+At the start of the guest, the kernel image is read and the ELF section is
+parsed. The hypervisor looks a specific section that has the string âXenâ
+(or âxenâ) to find out: where the kernel expects its virtual base address,
+what type of hypervisor it can work with, certain features the kernel image
+can support, and the location of the hypercall. There are two variants of this:
+
+; a). The lagacy ASCIIZ string with all of the keys concatenated. Each key
+being a string and the equal sign with the value also being an string
+(numeric values are typed as string). The delimiter is a comma.
+The key can be up to 32 characters and the value up to 128 characters.
+For example:
+
+       
GUEST_OS=Mini-OS,XEN_VER=xen-3.0,VIRT_BASE=0x0,ELF_PADDR_OFFSET=0x0,HYPERCALL_PAGE=0x2,LOADER=generic
+
+; b). A â.note.Xenâ section in ELF header conforming to the ELF .noteâ format.
+This structure is a 4-byte aligned structure. First section is an numerical
+key (aligned to 4 bytes); followed by either a string or a numerical value
+(again, aligned to 4 bytes). The values can up to any length, if the key is
+assumed to a string. If it is a numerical value, it is a long
+(64-bit value - which means 8 bytes).
+
+The parameters and its purpose are explained in
+[http://xenbits.xen.org/docs/unstable/hypercall/include,public,elfnote.h.html#incontents_elfnote
 here].
+
+And the XEN_ELF_FEATURES values are explained in
+[http://xenbits.xen.org/docs/unstable/hypercall/include,public,features.h.html#incontents_elfnote_features
 here].
+
+For example, if the ELF values were as so:
+
+{| border="1" cellpadding="2" cellspacing="0"
+ | Name of Xen ELF entry
+ | Contents
+ |-
+ | XEN_ELFNOTE_GUEST_OS (6)
+ | linux
+ |-
+ | XEN_ELFNOTE_GUEST_VERSION (7)
+ | 2.6
+ |-
+ | XEN_ELFNOTE_XEN_VERSION (5)
+ | xen-3.0
+ |-
+ | XEN_ELFNOTE_VIRT_BASE (3)
+ | 0xffffffff80000000
+ |-
+ | XEN_ELFNOTE_ENTRY (1)
+ | 0xffffffff81899200
+ |-
+ | XEN_ELFNOTE_HYPERCALL_PAGE (2)
+ | 0xffffffff81001000
+ |-
+ | XEN_ELFNOTE_FEATURES (10)
+ | !writable_page_tables|pae_pgdir_above_4gb
+ |-
+ | XEN_ELFNOTE_PAE_MODE (9)
+ | yes
+ |-
+ | XEN_ELFNOTE_LOADER (8)
+ | generic
+ |-
+ | XEN_ELFNOTE_SUSPEND_CANCEL (14)
+ | 1
+ |-
+ | XEN_ELFNOTE_HV_START_LOW
+ | 0xffff800000000000
+ |-
+ | XEN_ELFNOTE_PADDR_OFFSET
+ | 0
+ |}
+
+With that setup, the hypervisor constructs an initial page table that spans the
+region from virtual start address up (0xffffffff80000000) to the end of the
+p2m map.
+
+For example, the layout (which then is going to be feed in
+[http://xenbits.xen.org/docs/unstable/hypercall/include,public,xen.h.html#incontents_startofday]
+can look as so:
+
+{| border="1" cellpadding="2" cellspacing="0"
+ | Page Frame (PFN)
+ | contents
+ |-
+ | 0x0
+ | location of 
[http://xenbits.xen.org/docs/unstable/hypercall/include,public,xen.h.html#incontents_startofday_shared
 struct shared_info]
+ |-
+ | 0x1000
+ | location of the kernel
+ |-
+ | 0x1001
+ | location of the hypercall within the kernel
+ |-
+ | 0x1E3E
+ | ramdisk
+ |-
+ | 0xFC69
+ | phys2mach (P2M) - an array of machine frame numbers. The total size of this 
array is dependent on the nr_pages in
+[http://xenbits.xen.org/docs/unstable/hypercall/include,public,xen.h.html#incontents_startofday
 struct start_info]
+and the architecture of the guest (each entry is four bytes
+under 32-bit kernels and eight bytes under 64-bit kernels).
+ |-
+ | 0xFCE9
+ | location of 
+[http://xenbits.xen.org/docs/unstable/hypercall/include,public,xen.h.html#incontents_startofday
 start_info structure]
+ |-
+ | 0xFCEA
+ | location of
+[http://xenbits.xen.org/docs/unstable/hypercall/include,public,io,xs_wire.h.html#incontents_xenstore_struct
 XenStore structure].
+Also refer to XXX docs/misc/xenstore.txt
+ |-
+ | 0xFCEB
+ | Depending on the type of the guest (initial domain or subsequent domains), 
it can either be the
+[http://xenbits.xen.org/docs/unstable/hypercall/include,public,xen.h.html#incontents_startofday_dom0_console
 dom0_vga_console_info structure]
+or 
+[http://xenbits.xen.org/docs/unstable/hypercall/include,public,io,console.h.html
 XenConsole structure].
+The parameters defining this location are in the 
+[http://xenbits.xen.org/docs/unstable/hypercall/include,public,xen.h.html#incontents_startofday
 start_info structure]
+ |-
+ | 0xFCEC
+ | bootstrap page tables. These pagetables are loaded in the guest at startup
+and cover from 0x0 up to 0xFD6f (the bootstack).
+ |-
+ | 0xFD6F
+ | bootstrap stack.
+ |}
+
+In terms of virtual addresses, these example PFNs are mapped to virtual 
addresses
+as follow:
+{| border="1" cellpadding="2" cellspacing="0"
+ | Virtual Address
+ | contents
+ |-
+ | 0xffffffff80000000
+ | location of 
[http://xenbits.xen.org/docs/unstable/hypercall/include,public,xen.h.html#incontents_startofday_shared
 struct shared_info]
+ |-
+ | 0xffffffff81000000
+ | location of the kernel
+ |-
+ | 0xffffffff81001000
+ | location of the hypercall within the kernel
+ |-
+ | 0xffffffff81e3e000 
+ | ramdisk
+ |-
+ | 0xffffffff8fc69000
+ | phys2mach (P2M) - an array of machine frame numbers. The total size of this 
array is dependent on the nr_pages in
+[http://xenbits.xen.org/docs/unstable/hypercall/include,public,xen.h.html#incontents_startofday
 struct start_info]
+and the architecture of the guest (each entry is four bytes
+under 32-bit kernels and eight bytes under 64-bit kernels).
+ |-
+ | 0xffffffff8fce9000
+ | location of 
+[http://xenbits.xen.org/docs/unstable/hypercall/include,public,xen.h.html#incontents_startofday
 start_info structure]
+ |-
+ | 0xffffffff8fcea000
+ | location of
+[http://xenbits.xen.org/docs/unstable/hypercall/include,public,io,xs_wire.h.html#incontents_xenstore_struct
 XenStore structure].
+Also refer to XXX docs/misc/xenstore.txt
+ |-
+ | 0xffffffff8fceb000
+ | Depending on the type of the guest (initial domain or subsequent domains), 
it can either be the
+[http://xenbits.xen.org/docs/unstable/hypercall/include,public,xen.h.html#incontents_startofday_dom0_console
 dom0_vga_console_info structure]
+or 
+[http://xenbits.xen.org/docs/unstable/hypercall/include,public,io,console.h.html
 XenConsole structure].
+The parameters defining this location are in the 
+[http://xenbits.xen.org/docs/unstable/hypercall/include,public,xen.h.html#incontents_startofday
 start_info structure]
+ |-
+ | 0xffffffff8fcfc000
+ | bootstrap page tables. These pagetables are loaded in the guest at startup
+and cover from 0x0 up to 0xFD6f (the bootstack).
+ |-
+ | 0xffffffff8fd6f000
+ | bootstrap stack.
+ |}
+
+When the guest is launched, per
+[http://xenbits.xen.org/docs/unstable/hypercall/include,public,xen.h.html#incontents_startofday]
+explanation, the register %esi contains the virtual address to the
+start_info_t (0xffffffff8fce9000), the %cr3 points to the beginning of the
+bootstrap page-tables (0xffffffff8fcfc000), and the %esp points to the
+bootstrap stack (0xffffffff8fd6f000).
+
 = Virtual Address Space =
 
 Xen enforces certain restrictions on the virtual addresses which are


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