Since the Xen entry point into the kernel can be different from the
native entry point, a ELFNOTE is used in order to tell the
domain builder how to load and jump into the kernel entry point:
ELFNOTE(Xen, XEN_ELFNOTE_PHYS32_ENTRY, .long, xen_start32)The presence of the XEN_ELFNOTE_PHYS32_ENTRY note
indicates that the kernel supports the boot ABI described in this
document.
The domain builder must load the kernel into the guest memory space
and jump into the entry point defined at
XEN_ELFNOTE_PHYS32_ENTRY with the following machine
state:
ebx: contains the physical memory address where the
loader has placed the boot start info structure.
cr0: bit 0 (PE) must be set. All the other writeable
bits are cleared.
cr4: all bits are cleared.
cs: must be a 32-bit read/execute code segment with
a base of ‘0’ and a limit of ‘0xFFFFFFFF’. The selector value is
unspecified.
ds, es, ss: must be a
32-bit read/write data segment with a base of ‘0’ and a limit of
‘0xFFFFFFFF’. The selector values are all unspecified.
tr: must be a 32-bit TSS (active) with a base of ‘0’
and a limit of ‘0x67’.
eflags: bit 17 (VM) must be cleared. Bit 9 (IF) must
be cleared. Bit 8 (TF) must be cleared. Other bits are all
unspecified.
All other processor registers and flag bits are unspecified. The OS is in charge of setting up it’s own stack, GDT and IDT.
The format of the boot start info structure (pointed to by %ebx) can
be found in
xen/include/public/arch-x86/hvm/start_info.h
Other relevant information needed in order to boot a guest kernel (console page address, xenstore event channel…) can be obtained using HVMPARAMS, just like it’s done on HVM guests.
The setup of the hypercall page is also performed in the same way as HVM guests, using the hypervisor cpuid leaves and msr ranges.
AP startup can be performed using hypercalls or the local APIC if present. The following VCPU hypercalls can be used in order to bring up secondary vCPUs:
VCPUOP_initialise is used to set the initial state
of the vCPU. The argument passed to the hypercall must be of the type
vcpu_hvm_context. See public/hvm/hvm_vcpu.h for the layout
of the structure. Note that this hypercall allows starting the vCPU in
several modes (16/32/64bits), regardless of the mode the BSP is
currently running on.
VCPUOP_up is used to launch the vCPU once the
initial state has been set using
VCPUOP_initialise.
VCPUOP_down is used to bring down a vCPU.
VCPUOP_is_up is used to scan the number of available
vCPUs.
PVH guests that have access to hardware (either emulated or real)
will also have ACPI tables with the description of the hardware that’s
available to the guest. This applies to both privileged and unprivileged
guests. A pointer to the position of the RSDP in memory (if present) can
be fetched from the start info structure that’s passed at boot time
(field rsdp_paddr).
Description of paravirtualized devices will come from XenStore, just as it’s done for HVM guests.
Interrupts from physical devices are delivered using native methods, this is done in order to take advantage of new hardware assisted virtualization functions, like posted interrupts. This implies that PVH guests with physical devices will also have the necessary interrupt controllers in order to manage the delivery of interrupts from those devices, using the same interfaces that are available on native hardware.
Interrupts from paravirtualized devices are delivered using event
channels, see [Event Channel Internals][event_channels] for more
detailed information about event channels. Delivery of those interrupts
can be configured in the same way as HVM guests, check
xen/include/public/hvm/params.h and
xen/include/public/hvm/hvm_op.h for more information about
available delivery methods.
PVH guests are currently booted with the default MTRR type set to write-back and MTRR enabled. This allows DomUs to start with a sane MTRR state. Note that this will have to be revisited when pci-passthrough is added to PVH in order to set MMIO regions as UC.
Xen guarantees that RAM regions will always have the WB cache type set in the initial MTRR state, either set by the default MTRR type or by other means.
A PVH hardware domain is booted with the same MTRR state as the one found on the host. This is done because the hardware domain memory map is already a modified copy of the host memory map, so the same MTRR setup should work.