Re: [PATCH v9 14/15] docs: Add protvirt docs

[Claudio Imbrenda <imbrenda@linux.ibm.com>]

On Wed, 11 Mar 2020 09:21:50 -0400
Janosch Frank <frankja@linux.ibm.com> wrote:

> Lets add some documentation for the Protected VM functionality.
> 
> Signed-off-by: Janosch Frank <frankja@linux.ibm.com>
> ---
>  docs/system/index.rst    |  1 +
>  docs/system/protvirt.rst | 56
> ++++++++++++++++++++++++++++++++++++++++ 2 files changed, 57
> insertions(+) create mode 100644 docs/system/protvirt.rst
> 
> diff --git a/docs/system/index.rst b/docs/system/index.rst
> index 6e5f20fa1333ce23..74afbd7cc3fc0296 100644
> --- a/docs/system/index.rst
> +++ b/docs/system/index.rst
> @@ -34,3 +34,4 @@ Contents:
>     deprecated
>     build-platforms
>     license
> +   protvirt
> diff --git a/docs/system/protvirt.rst b/docs/system/protvirt.rst
> new file mode 100644
> index 0000000000000000..6c8cf0f7910eae86
> --- /dev/null
> +++ b/docs/system/protvirt.rst
> @@ -0,0 +1,56 @@
> +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
> +which is 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 kernel command line.

I'd add "of the host" just to make it extra clear

> +
> +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
> +`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). Stag3 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 via 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 by genprotimg of the s390-tools package.

Reviewed-by: Claudio Imbrenda <imbrenda@linux.ibm.com>