Re: [PATCH v10 15/16] docs: system: Add protvirt docs

[Christian Borntraeger]

On 18.03.20 15:30, Janosch Frank wrote:
> Let's add some documentation for the Protected VM functionality.
> 
> Signed-off-by: Janosch Frank <address@hidden>
> Acked-by: David Hildenbrand <address@hidden>
> Reviewed-by: Claudio Imbrenda <address@hidden>

Acked-by: Christian Borntraeger <address@hidden>

> ---
>  docs/system/s390x/protvirt.rst | 59 ++++++++++++++++++++++++++++++++++
>  docs/system/target-s390x.rst   |  5 +++
>  2 files changed, 64 insertions(+)
>  create mode 100644 docs/system/s390x/protvirt.rst
> 
> diff --git a/docs/system/s390x/protvirt.rst b/docs/system/s390x/protvirt.rst
> new file mode 100644
> index 0000000000000000..71172511632f7bcb
> --- /dev/null
> +++ b/docs/system/s390x/protvirt.rst
> @@ -0,0 +1,59 @@
> +Protected Virtualization on s390x
> +=================================
> +
> +The memory and most of the registers of Protected Virtual Machines
> +(PVMs) are encrypted or inaccessible to the hypervisor, effectively
> +prohibiting VM introspection when the VM is running. At rest, PVMs are
> +encrypted and can only be decrypted by the firmware, represented by an
> +entity called Ultravisor, of specific IBM Z machines.
> +
> +
> +Prerequisites
> +-------------
> +
> +To run PVMs, a machine with the Protected Virtualization feature, as
> +indicated by the Ultravisor Call facility (stfle bit 158), is
> +required. The Ultravisor needs to be initialized at boot by setting
> +`prot_virt=1` on the host's kernel command line.
> +
> +Running PVMs requires using the KVM hypervisor.
> +
> +If those requirements are met, the capability `KVM_CAP_S390_PROTECTED`
> +will indicate that KVM can support PVMs on that LPAR.
> +
> +
> +QEMU Settings
> +-------------
> +
> +To indicate to the VM that it can transition into protected mode, the
> +`Unpack facility` (stfle bit 161 represented by the feature
> +`unpack`/`S390_FEAT_UNPACK`) needs to be part of the cpu model of
> +the VM.
> +
> +All I/O devices need to use the IOMMU.
> +Passthrough (vfio) devices are currently not supported.
> +
> +Host huge page backings are not supported. However guests can use huge
> +pages as indicated by its facilities.
> +
> +
> +Boot Process
> +------------
> +
> +A secure guest image can either be loaded from disk or supplied on the
> +QEMU command line. Booting from disk is done by the unmodified
> +s390-ccw BIOS. I.e., the bootmap is interpreted, multiple components
> +are read into memory and control is transferred to one of the
> +components (zipl stage3). Stage3 does some fixups and then transfers
> +control to some program residing in guest memory, which is normally
> +the OS kernel. The secure image has another component prepended
> +(stage3a) that uses the new diag308 subcodes 8 and 10 to trigger the
> +transition into secure mode.
> +
> +Booting from the image supplied on the QEMU command line requires that
> +the file passed via -kernel has the same memory layout as would result
> +from the disk boot. This memory layout includes the encrypted
> +components (kernel, initrd, cmdline), the stage3a loader and
> +metadata. In case this boot method is used, the command line
> +options -initrd and -cmdline are ineffective. The preparation of a PVM
> +image is done via the `genprotimg` tool from the s390-tools package.
> diff --git a/docs/system/target-s390x.rst b/docs/system/target-s390x.rst
> index 4c8b7cdd66154d26..8d97158dc804af1c 100644
> --- a/docs/system/target-s390x.rst
> +++ b/docs/system/target-s390x.rst
> @@ -24,3 +24,8 @@ or vfio-ap is also available.
>  .. toctree::
>     s390x/vfio-ap
>  
> +Architectural features
> +~~~~~~~~
> +
> +.. toctree::
> +   s390x/protvirt
>