Re: [Xen-devel] [PATCH] docs/sphinx: How Xen Boots on x86

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 09/12/2019 15:20, Jan Beulich wrote:
> On 06.12.2019 20:34, Andrew Cooper wrote:
>> Begin to document how the x86 build of Xen boots.  It is by no means 
>> complete,
>> but is a start.
>>
>> Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
>> ---
>> CC: Jan Beulich <JBeulich@xxxxxxxx>
>> CC: Wei Liu <wl@xxxxxxx>
>> CC: Roger Pau Monné <roger.pau@xxxxxxxxxx>
>>
>> This came about while I sat in SFO waiting for a delayed flight, and was 
>> asked
>> a question by the Trenchboot folk.
>>
>> Writing it down like this already highlights some issues, such as the EFI
>> binary having MB1/MB2 headers.
> While at least the MB1 ones aren't really necessary, they also don't
> do any harm, do they?

At least one version of iPXE seems to prefer MB2 over PE, which is why I
was asked "where are Xen's entrypoints?" in the first place.

>
>> --- /dev/null
>> +++ b/docs/hypervisor-guide/x86/how-xen-boots.rst
>> @@ -0,0 +1,101 @@
>> +.. SPDX-License-Identifier: CC-BY-4.0
>> +
>> +How Xen Boots
>> +=============
>> +
>> +This is an at-a-glance reference of Xen's booting capabilities and
>> +expectations.
>> +
>> +
>> +Build
>> +-----
>> +
>> +A build of xen produces ``xen.gz`` and optionally ``xen.efi`` as final
>> +artefacts.
>> +
>> + * For BIOS, Xen supports the Multiboot 1 and 2 protocols.
>> +
>> + * For EFI, Xen supports Multiboot 2 with EFI extensions, and native EFI64.
>> +
>> + * For virtualisation, Xen supports starting directly with the PVH boot
>> +   protocol.
>> +
>> +
>> +Objects
>> +~~~~~~~
>> +
>> +To begin with, most object files are compiled and linked.  This includes the
>> +Multiboot 1 and 2 headers and entrypoints, including the Multiboot 2 tags 
>> for
>> +EFI extensions.  When ``CONFIG_PVH_GUEST`` is selected at build time, this
>> +includes the PVH entrypoint and associated ELF notes.
>> +
>> +Depending on whether the compiler supports ``__attribute__((__ms_abi__))`` 
>> or
>> +not, either an EFI stub is included which nops/fails applicable setup calls,
>> +or full EFI support is included.
> Perhaps also mention that the linker needs to support the necessary
> binary output format? And perhaps "setup and runtime calls"?

Link time behaviour is (deliberately) in a later section.

>
>> +Protocols and entrypoints
>> +~~~~~~~~~~~~~~~~~~~~~~~~~
>> +
>> +All headers and tags are built in ``xen/arch/x86/boot/head.S``
>> +
>> +The Multiboot 1 headers request aligned modules and memory information.  
>> Entry
>> +is via the start of the binary image, which is the ``start`` symbol.  This
>> +entrypoint must be started in 32bit mode.
>> +
>> +The Multiboot 2 headers are more flexible, and in addition request that the
>> +image be loaded as high as possible below the 4G boundary, with 2M 
>> alignment.
>> +Entry is still via the ``start`` symbol as with MB1.
> Perhaps explicitly (re)state this is in 32-bit mode?
>
>> +Headers for the EFI MB2 extensions are also present.  These request that
>> +``ExitBootServices()`` not be called, and register ``__efi_mb2_start`` as an
>> +alternative entrypoint, entered in 64bit mode.
>> +
>> +If ``CONFIG_PVH_GUEST`` was selected at build time, an Elf note is included
>> +which indicates the ability to use the PVH boot protocol, and registers
>> +``__pvh_start`` as the entrypoint, entered in 32bit mode.
>> +
>> +
>> +xen.gz
>> +~~~~~~
>> +
>> +The objects are linked together to form ``xen-syms`` which is an ELF64
>> +executable with full debugging symbols.  ``xen.gz`` is formed by stripping
>> +``xen-syms``, then repackaging the result as an ELF32 object with a single
>> +load section at 2MB, and ``gzip``-ing the result.  Despite the ELF32 having 
>> a
>> +fixed load address, its contents are relocatable.
> This is a little ambiguous I guess - most of the code is PIC and as
> such relocatable, but not in a way a boot loader could arrange for.

I don't follow your concern.

Everything which needs to be is position independent (subject to being
loaded on a 2M boundary IIRC), and this property is requested by the MB2
header.

>
>> +Any bootloader which unzips the binary and follows the ELF headers will 
>> place
>> +it at the 2M boundary and jump to ``start`` which is the identified entry
>> +point.  However, Xen depends on being entered with the MB1 or MB2 protocols,
>> +and will terminate otherwise.
>> +
>> +The MB2+EFI entrypoint depends on being entered with the MB2 protocol, and
>> +will terminate if the entry protocol is wrong, or if EFI details aren't
>> +provided, or if EFI Boot Services are not available.
>> +
>> +
>> +xen.efi
>> +~~~~~~~
>> +
>> +When a PEI-capable toolchain is found, the objects are linked together and a
>> +PE64 binary is created.  It can be run directly from the EFI shell, and has
> I think it's commonly called PE32+, not PE64.

Ok., because by definition, it can stack.

>
> Maybe also mention the "chainloader" grub command it can be used with?
> Or do we consider this uninteresting enough with modern grub?

I wasn't planning to consider chainloading, as it isn't really relevant
to how Xen starts, and can be stacked in many more interesting ways than
usefully be described.

~Andrew

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