# 1 x86/HVM direct boot ABI

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.

## 1.1 AP startup

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.

## 1.2 Hardware description

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.

## 1.3 Interrupts

### 1.3.1 Interrupts from physical devices

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.

### 1.3.2 Interrupts from paravirtualized devices

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.

## 1.4 MTRR

### 1.4.1 Unprivileged guests

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.

### 1.4.2 Hardware domain

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.