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Re: [Xen-devel] RFC: very initial PVH design document

On Fri, 22 Aug 2014 16:55:08 +0200
Roger Pau Monnà <roger.pau@xxxxxxxxxx> wrote:

> Hello,
> I've started writing a document in order to describe the interface 
> exposed by Xen to PVH guests, and how it should be used (by guest 
> OSes). The document is far from complete (see the amount of TODOs 
> scattered around), but given the lack of documentation regarding PVH
> I think it's a good starting point. The aim of this is that it should
> be committed to the Xen repository once it's ready. Given that this
> is still a *very* early version I'm not even posting it as a patch.
> Please comment, and try to fill the holes if possible ;).
> Roger.
> ---
> # PVH Specification #
> ## Rationale ##
> 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.
> ## Early boot ##
> 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")
> __XSTRING(__FreeBSD_version)) ELFNOTE(Xen,
> XEN_ELFNOTE_XEN_VERSION,    .asciz, "xen-3.0") ELFNOTE(Xen,
> XEN_ELFNOTE_ENTRY,          .quad,  xen_start) ELFNOTE(Xen,
> XEN_ELFNOTE_HYPERCALL_PAGE, .quad,  hypercall_page) ELFNOTE(Xen,
> "writable_descriptor_tables|auto_translated_physmap|supervisor_mode_kernel|hvm_callback_vector")
> ELFNOTE(Xen, XEN_ELFNOTE_PAE_MODE,       .asciz, "yes") ELFNOTE(Xen,
> XEN_ELFNOTE_LOADER,         .asciz, "generic") ELFNOTE(Xen,
> XEN_ELFNOTE_BSD_SYMTAB,     .asciz, "yes")

It will be helpful to add:

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 memory address of the kernel
> entry point.
>   * XEN_ELFNOTE_HYPERCALL_PAGE: contains the memory address of the
> hypercall 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 this case 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.

Hmm... for linux I'd word that as follows:

A PVH guest is started by specifying pvh=1 in the config file. However,
for the guest to be launched as a PVH guest, it must minimally advertise 
certain features which are: auto_translated_physmap, hvm_callback_vector, 
writable_descriptor_tables, and supervisor_mode_kernel. This is done
linux:arch/x86/xen/xen-head.S for more info. A list of all xen features
can be found in xen:include/public/features.h. However, at present
the absence of these features does not make it automatically boot in PV
mode, but that may change in future. The ultimate goal is, if a guest
supports these features, then boot it automatically in PVH mode, otherwise
boot it in PV mode.

[You can leave out the last part if you want, or just take whatever from

> Xen will jump into the kernel entry point defined in
> `XEN_ELFNOTE_ENTRY` with paging enabled (either long or protected
> mode depending on the kernel bitness) and some basic page tables
> setup.

If I may rephrase:

Guest is launched at the entry point specified in XEN_ELFNOTE_ENTRY
with paging, PAE, and long mode enabled. At present only 64bit mode
is supported, however, in future compat mode support will be added.
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 were Xen has placed the start_info structure.
> The `rsp` (`esp` on 32bits) will contain a 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 on the `xen.h` public header.

Since the above is all true for PV guest, you could begin it with:

Just like a PV guest, the rsi ....

> ### Initial amd64 control registers values ###
> 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.
> On `CR0` the following bits are set by Xen:
>   * PE (bit 0): protected mode enable.
>   * ET (bit 4): 80387 external math coprocessor.
>   * PG (bit 31): paging enabled.
> On `CR4` the following bits are set by Xen:
>   * PAE (bit 5): PAE enabled.
> And finally on `EFER` the following features are enabled:
>   * LME (bit 8): Long mode enable.
>   * LMA (bit 10): Long mode active.
> *TODO*: do we expect this flags to change? Are there other flags that
> might be enabled depending on the hardware we are running on?

Can't think of anything...

> ## Memory ##
> 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.

Do you wanna expand a bit since this is another big distinction from
a PV guest?

which means that page tables are native and guest managed. 
This also implies that mmu_update hypercall is not available to a PVH
guest, unlike a PV guest.  The guest is configured at start so it can 
access all pages upto start_info->nr_pages.

> 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.

We map all non-ram regions for dom0 1:1 till the highest non-ram e820
entry. If there is anything that is beyond the last e820 entry,
it will remain unmapped.

Correct, passthru needs to be figured.

> 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.
> ## Physical devices ##
> 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 of parsing the ACPI tables
> and notify 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.
> ## Interrupts ##
> All interrupts on PVH guests are routed over event channels, see
> [Event Channel Internals][event_channels] 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
>     value = (0x2 << 56) | vector_value
> 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.
> *TODO*: provide a reference about how to interact with FIFO event
> channels?
> ### Interrupts from physical devices ###
> 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 interrupt routing ###
> 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
> After this hypercall, `PHYSDEVOP_alloc_irq_vector` is used to
> allocate a vector:
>     irq = 9
>     vector = 0
> *TODO*: I'm not sure why we need those two hypercalls, and it's usage
> is not documented anywhere. Need to clarify what the parameters mean
> and what effect they have.
> 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
> overwrites found on the ACPI tables and notify Xen about them.
> ### MSI ###
> 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
>     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`.
> ### MSI-X ###
> *TODO*: how to register/use them.
> ## Event timers and timecounters ##
> 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`.

All the above is great info, not PVH specific tho. May wanna mention
it fwiw. 

> ## SMP discover and bring up ##
> 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. The most relevant ones are `rip`
> and `rsp` in order to set the start address and the stack.
>   * `ctrlreg[3]`: contains the address of the page tables that will
> be used by the vCPU.
> 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.

If you want, you could put linux reference here:

For an example, please see cpu_initialize_context() in arch/x86/xen/smp.c
in linux.

> ## Control operations (reboot/shutdown) ##
> 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)
> ## CPUID ##
> *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.
> ## Final notes ##
> All the other hardware functionality not described in this document
> should be assumed to be performed in the same way as native.
> [evnet_channels]: http://wiki.xen.org/wiki/Event_Channel_Internals

Great work Roger! Thanks a lot for writing it.


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