PVH is a new kind of guest that has been introduced on Xen 4.4 as a DomU, and on Xen 4.5 as a Dom0. The aim of PVH is to make use of the hardware virtualization extensions present in modern x86 CPUs in order to improve performance.
PVH is considered a mix between PV and HVM, and can be seen as a PV guest that runs inside of an HVM container, or as a PVHVM guest without any emulated devices. The design goal of PVH is to provide the best performance possible and to reduce the amount of modifications needed for a guest OS to run in this mode (compared to pure PV).
This document tries to describe the interfaces used by PVH guests, focusing on how an OS should make use of them in order to support PVH.
PVH guests use the PV boot mechanism, that means that the kernel is loaded and directly launched by Xen (by jumping into the entry point). In order to do this Xen ELF Notes need to be added to the guest kernel, so that they contain the information needed by Xen. Here is an example of the ELF Notes added to the FreeBSD amd64 kernel in order to boot as PVH:
ELFNOTE(Xen, XEN_ELFNOTE_GUEST_OS, .asciz, "FreeBSD")
ELFNOTE(Xen, XEN_ELFNOTE_GUEST_VERSION, .asciz, __XSTRING(__FreeBSD_version))
ELFNOTE(Xen, XEN_ELFNOTE_XEN_VERSION, .asciz, "xen-3.0")
ELFNOTE(Xen, XEN_ELFNOTE_VIRT_BASE, .quad, KERNBASE)
ELFNOTE(Xen, XEN_ELFNOTE_PADDR_OFFSET, .quad, KERNBASE)
ELFNOTE(Xen, XEN_ELFNOTE_ENTRY, .quad, xen_start)
ELFNOTE(Xen, XEN_ELFNOTE_HYPERCALL_PAGE, .quad, hypercall_page)
ELFNOTE(Xen, XEN_ELFNOTE_HV_START_LOW, .quad, HYPERVISOR_VIRT_START)
ELFNOTE(Xen, XEN_ELFNOTE_FEATURES, .asciz, "writable_descriptor_tables|auto_translated_physmap|supervisor_mode_kernel|hvm_callback_vector")
ELFNOTE(Xen, XEN_ELFNOTE_PAE_MODE, .asciz, "yes")
ELFNOTE(Xen, XEN_ELFNOTE_L1_MFN_VALID, .long, PG_V, PG_V)
ELFNOTE(Xen, XEN_ELFNOTE_LOADER, .asciz, "generic")
ELFNOTE(Xen, XEN_ELFNOTE_SUSPEND_CANCEL, .long, 0)
ELFNOTE(Xen, XEN_ELFNOTE_BSD_SYMTAB, .asciz, "yes")
On the Linux side, the above can be found in arch/x86/xen/xen-head.S.
It is important to highlight the following notes:
XEN_ELFNOTE_ENTRY: contains the virtual memory address of the kernel entry
point.XEN_ELFNOTE_HYPERCALL_PAGE: contains the virtual memory address of the
hypercal page inside of the guest kernel (this memory region will be filled
by Xen prior to booting).XEN_ELFNOTE_FEATURES: contains the list of features supported by the kernel.
In the example above the kernel is only able to boot as a PVH guest, but
those options can be mixed with the ones used by pure PV guests in order to
have a kernel that supports both PV and PVH (like Linux). The list of
options available can be found in the features.h public header. Note that
in the example above hvm_callback_vector is in XEN_ELFNOTE_FEATURES.
Older hypervisors will balk at this being part of it, so it can also be put
in XEN_ELFNOTE_SUPPORTED_FEATURES which older hypervisors will ignore.Xen will jump into the kernel entry point defined in XEN_ELFNOTE_ENTRY with
paging enabled (either long mode or protected mode with paging turned on
depending on the kernel bitness) and some basic page tables setup. An important
distinction for a 64bit PVH is that it is launched at privilege level 0 as
opposed to a 64bit PV guest which is launched at privilege level 3.
Also, the rsi (esi on 32bits) register is going to contain the virtual
memory address where Xen has placed the start_info structure. The rsp (esp
on 32bits) will point to the top of an initial single page stack, that can be
used by the guest kernel. The start_info structure contains all the info the
guest needs in order to initialize. More information about the contents can be
found in the xen.h public header.
Initial values for the control registers are set up by Xen before booting the guest kernel. The guest kernel can expect to find the following features enabled by Xen.
CR0 has the following bits set by Xen:
CR4 has the following bits set by Xen:
And finally in EFER the following features are enabled:
At least the following flags in EFER are guaranteed to be disabled:
There's no guarantee about the state of the other bits in the EFER register.
All the segments selectors are set with a flat base at zero.
The cs segment selector attributes are set to 0x0a09b, which describes an
executable and readable code segment only accessible by the most privileged
level. The segment is also set as a 64-bit code segment (L flag set, D flag
unset).
The remaining segment selectors (ds, ss, es, fs and gs) are all set
to the same values. The attributes are set to 0x0c093, which implies a read and
write data segment only accessible by the most privileged level.
The FS.base, GS.base and KERNEL_GS.base MSRs are zeroed out.
The IDT and GDT are also zeroed, so the guest must be specially careful to
not trigger a fault until after they have been properly set. The way of setting
the IDT and the GDT is using the native instructions as would be done on bare
metal.
The RFLAGS register is guaranteed to be clear when jumping into the kernel
entry point, with the exception of the reserved bit 1 set.
Since PVH guests rely on virtualization extensions provided by the CPU, they have access to a hardware virtualized MMU, which means page-table related operations should use the same instructions used on native.
There are however some differences with native. The usage of native MTRR
operations is forbidden, and XENPF_*_memtype hypercalls should be used
instead. This can be avoided by simply not using MTRR and setting all the
memory attributes using PAT, which doesn't require the usage of any hypercalls.
Since PVH doesn't use a BIOS in order to boot, the physical memory map has
to be retrieved using the XENMEM_memory_map hypercall, which will return
an e820 map. This memory map might contain holes that describe MMIO regions,
that will be already setup by Xen.
TODO: we need to figure out what to do with MMIO regions, right now Xen sets all the holes in the native e820 to MMIO regions for Dom0 up to 4GB. We need to decide what to do with MMIO regions above 4GB on Dom0, and what to do for PVH DomUs with pci-passthrough.
In the case of a guest started with memory != maxmem, the e820 memory map
returned by Xen will contain the memory up to maxmem. The guest has to be very
careful to only use the lower memory pages up to the value contained in
start_info->nr_pages because any memory page above that value will not be
populated.
When running as Dom0 the guest OS has the ability to interact with the physical devices present in the system. A note should be made that PVH guests require a working IOMMU in order to interact with physical devices.
The first step in order to manipulate the devices is to make Xen aware of
them. Due to the fact that all the hardware description on x86 comes from
ACPI, Dom0 is responsible for parsing the ACPI tables and notifying Xen about
the devices it finds. This is done with the PHYSDEVOP_pci_device_add
hypercall.
TODO: explain the way to register the different kinds of PCI devices, like devices with virtual functions.
All interrupts on PVH guests are routed over event channels, see Event Channel Internals for more detailed information about event channels. In order to inject interrupts into the guest an IDT vector is used. This is the same mechanism used on PVHVM guests, and allows having per-cpu interrupts that can be used to deliver timers or IPIs.
In order to register the callback IDT vector the HVMOP_set_param hypercall
is used with the following values:
domid = DOMID_SELF
index = HVM_PARAM_CALLBACK_IRQ
value = (0x2 << 56) | vector_value
The OS has to program the IDT for the vector_value using the baremetal
mechanism.
In order to know which event channel has fired, we need to look into the
information provided in the shared_info structure. The evtchn_pending
array is used as a bitmap in order to find out which event channel has
fired. Event channels can also be masked by setting it's port value in the
shared_info->evtchn_mask bitmap.
When running as Dom0 (or when using pci-passthrough) interrupts from physical devices are routed over event channels. There are 3 different kind of physical interrupts that can be routed over event channels by Xen: IO APIC, MSI and MSI-X interrupts.
Since physical interrupts usually need EOI (End Of Interrupt), Xen allows the
registration of a memory region that will contain whether a physical interrupt
needs EOI from the guest or not. This is done with the
PHYSDEVOP_pirq_eoi_gmfn_v2 hypercall that takes a parameter containing the
physical address of the memory page that will act as a bitmap. Then in order to
find out if an IRQ needs EOI or not, the OS can perform a simple bit test on the
memory page using the PIRQ value.
IO APIC interrupts can be routed over event channels using PHYSDEVOP
hypercalls. First the IRQ is registered using the PHYSDEVOP_map_pirq
hypercall, as an example IRQ#9 is used here:
domid = DOMID_SELF
type = MAP_PIRQ_TYPE_GSI
index = 9
pirq = 9
The IRQ#9 is now registered as PIRQ#9. The triggering and polarity can also
be configured using the PHYSDEVOP_setup_gsi hypercall:
gsi = 9 # This is the IRQ value.
triggering = 0
polarity = 0
In this example the IRQ would be configured to use edge triggering and high polarity.
Finally the PIRQ can be bound to an event channel using the
EVTCHNOP_bind_pirq, that will return the event channel port the PIRQ has been
assigned. After this the event channel will be ready for delivery.
NOTE: when running as Dom0, the guest has to parse the interrupt overrides found on the ACPI tables and notify Xen about them.
In order to configure MSI interrupts for a device, Xen must be made aware of
it's presence first by using the PHYSDEVOP_pci_device_add as described above.
Then the PHYSDEVOP_map_pirq hypercall is used:
domid = DOMID_SELF
type = MAP_PIRQ_TYPE_MSI_SEG or MAP_PIRQ_TYPE_MULTI_MSI
index = -1
pirq = -1
bus = pci_device_bus
devfn = pci_device_function
entry_nr = number of MSI interrupts
The type has to be set to MAP_PIRQ_TYPE_MSI_SEG if only one MSI interrupt
source is being configured. On devices that support MSI interrupt groups
MAP_PIRQ_TYPE_MULTI_MSI can be used to configure them by also placing the
number of MSI interrupts in the entry_nr field.
The values in the bus and devfn field should be the same as the ones used
when registering the device with PHYSDEVOP_pci_device_add.
TODO: how to register/use them.
Since some hardware is not available on PVH (like the local APIC), Xen provides the OS with suitable replacements in order to get the same functionality. One of them is the timer interface. Using a set of hypercalls, a guest OS can set event timers that will deliver and event channel interrupt to the guest.
In order to use the timer provided by Xen the guest OS first needs to register
a VIRQ event channel to be used by the timer to deliver the interrupts. The
event channel is registered using the EVTCHNOP_bind_virq hypercall, that
only takes two parameters:
virq = VIRQ_TIMER
vcpu = vcpu_id
The port that's going to be used by Xen in order to deliver the interrupt is
returned in the port field. Once the interrupt is set, the timer can be
programmed using the VCPUOP_set_singleshot_timer hypercall.
flags = VCPU_SSHOTTMR_future
timeout_abs_ns = absolute value when the timer should fire
It is important to notice that the VCPUOP_set_singleshot_timer hypercall must
be executed from the same vCPU where the timer should fire, or else Xen will
refuse to set it. This is a single-shot timer, so it must be set by the OS
every time it fires if a periodic timer is desired.
Xen also shares a memory region with the guest OS that contains time related
values that are updated periodically. This values can be used to implement a
timecounter or to obtain the current time. This information is placed inside of
shared_info->vcpu_info[vcpu_id].time. The uptime (time since the guest has
been launched) can be calculated using the following expression and the values
stored in the vcpu_time_info struct:
system_time + ((((tsc - tsc_timestamp) << tsc_shift) * tsc_to_system_mul) >> 32)
The timeout that is passed to VCPUOP_set_singleshot_timer has to be
calculated using the above value, plus the timeout the system wants to set.
If the OS also wants to obtain the current wallclock time, the value calculated
above has to be added to the values found in shared_info->wc_sec and
shared_info->wc_nsec.
The process of bringing up secondary CPUs is obviously different from native,
since PVH doesn't have a local APIC. The first thing to do is to figure out
how many vCPUs the guest has. This is done using the VCPUOP_is_up hypercall,
using for example this simple loop:
for (i = 0; i < MAXCPU; i++) {
ret = HYPERVISOR_vcpu_op(VCPUOP_is_up, i, NULL);
if (ret >= 0)
/* vCPU#i is present */
}
Note than when running as Dom0, the ACPI tables might report a different number of available CPUs. This is because the value on the ACPI tables is the number of physical CPUs the host has, and it might bear no resemblance with the number of vCPUs Dom0 actually has so it should be ignored.
In order to bring up the secondary vCPUs they must be configured first. This is
achieved using the VCPUOP_initialise hypercall. A valid context has to be
passed to the vCPU in order to boot. The relevant fields for PVH guests are
the following:
flags: contains VGCF_* flags (see arch-x86/xen.h public header).user_regs: struct that contains the register values that will be set on
the vCPU before booting. All GPRs are available to be set, however, the
most relevant ones are rip and rsp in order to set the start address
and the stack. Please note, all selectors must be null.ctrlreg[3]: contains the address of the page tables that will be used by
the vCPU. Other control registers should be set to zero, or else the
hypercall will fail with -EINVAL.After the vCPU is initialized with the proper values, it can be started by
using the VCPUOP_up hypercall. The values of the other control registers of
the vCPU will be the same as the ones described in the control registers
section.
Examples about how to bring up secondary CPUs can be found on the FreeBSD
code base in sys/x86/xen/pv.c and on Linux arch/x86/xen/smp.c.
Reboot and shutdown operations on PVH guests are performed using hypercalls.
In order to issue a reboot, a guest must use the SHUTDOWN_reboot hypercall.
In order to perform a power off from a guest DomU, the SHUTDOWN_poweroff
hypercall should be used.
The way to perform a full system power off from Dom0 is different than what's
done in a DomU guest. In order to perform a power off from Dom0 the native
ACPI path should be followed, but the guest should not write the SLP_EN
bit to the Pm1Control register. Instead the XENPF_enter_acpi_sleep hypercall
should be used, filling the following data in the xen_platform_op struct:
cmd = XENPF_enter_acpi_sleep
interface_version = XENPF_INTERFACE_VERSION
u.enter_acpi_sleep.pm1a_cnt_val = Pm1aControlValue
u.enter_acpi_sleep.pm1b_cnt_val = Pm1bControlValue
This will allow Xen to do it's clean up and to power off the system. If the host is using hardware reduced ACPI, the following field should also be set:
u.enter_acpi_sleep.flags = XENPF_ACPI_SLEEP_EXTENDED (0x1)
The cpuid instruction that should be used is the normal cpuid, not the
emulated cpuid that PV guests usually require.
TDOD: describe which cpuid flags a guest should ignore and also which flags describe features can be used. It would also be good to describe the set of cpuid flags that will always be present when running as PVH.
All the other hardware functionality not described in this document should be assumed to be performed in the same way as native.