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