1 .. SPDX-License-Identifier: GPL-2.0 2 3 =========================================== 4 s390 (IBM Z) Protected Virtualization dumps 5 =========================================== 6 7 Summary 8 ------- 9 10 Dumping a VM is an essential tool for debugging problems inside 11 it. This is especially true when a protected VM runs into trouble as 12 there's no way to access its memory and registers from the outside 13 while it's running. 14 15 However when dumping a protected VM we need to maintain its 16 confidentiality until the dump is in the hands of the VM owner who 17 should be the only one capable of analysing it. 18 19 The confidentiality of the VM dump is ensured by the Ultravisor who 20 provides an interface to KVM over which encrypted CPU and memory data 21 can be requested. The encryption is based on the Customer 22 Communication Key which is the key that's used to encrypt VM data in a 23 way that the customer is able to decrypt. 24 25 26 Dump process 27 ------------ 28 29 A dump is done in 3 steps: 30 31 **Initiation** 32 33 This step initializes the dump process, generates cryptographic seeds 34 and extracts dump keys with which the VM dump data will be encrypted. 35 36 **Data gathering** 37 38 Currently there are two types of data that can be gathered from a VM: 39 the memory and the vcpu state. 40 41 The vcpu state contains all the important registers, general, floating 42 point, vector, control and tod/timers of a vcpu. The vcpu dump can 43 contain incomplete data if a vcpu is dumped while an instruction is 44 emulated with help of the hypervisor. This is indicated by a flag bit 45 in the dump data. For the same reason it is very important to not only 46 write out the encrypted vcpu state, but also the unencrypted state 47 from the hypervisor. 48 49 The memory state is further divided into the encrypted memory and its 50 metadata comprised of the encryption tweaks and status flags. The 51 encrypted memory can simply be read once it has been exported. The 52 time of the export does not matter as no re-encryption is 53 needed. Memory that has been swapped out and hence was exported can be 54 read from the swap and written to the dump target without need for any 55 special actions. 56 57 The tweaks / status flags for the exported pages need to be requested 58 from the Ultravisor. 59 60 **Finalization** 61 62 The finalization step will provide the data needed to be able to 63 decrypt the vcpu and memory data and end the dump process. When this 64 step completes successfully a new dump initiation can be started.
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