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diff --git a/docs/devel/nested-papr.txt b/docs/devel/nested-papr.txt new file mode 100644 index 0000000..9094365 --- /dev/null +++ b/docs/devel/nested-papr.txt @@ -0,0 +1,119 @@ +Nested PAPR API (aka KVM on PowerVM) +==================================== + +This API aims at providing support to enable nested virtualization with +KVM on PowerVM. While the existing support for nested KVM on PowerNV was +introduced with cap-nested-hv option, however, with a slight design change, +to enable this on papr/pseries, a new cap-nested-papr option is added. eg: + + qemu-system-ppc64 -cpu POWER10 -machine pseries,cap-nested-papr=true ... + +Work by: + Michael Neuling <mikey@neuling.org> + Vaibhav Jain <vaibhav@linux.ibm.com> + Jordan Niethe <jniethe5@gmail.com> + Harsh Prateek Bora <harshpb@linux.ibm.com> + Shivaprasad G Bhat <sbhat@linux.ibm.com> + Kautuk Consul <kconsul@linux.vnet.ibm.com> + +Below taken from the kernel documentation: + +Introduction +============ + +This document explains how a guest operating system can act as a +hypervisor and run nested guests through the use of hypercalls, if the +hypervisor has implemented them. The terms L0, L1, and L2 are used to +refer to different software entities. L0 is the hypervisor mode entity +that would normally be called the "host" or "hypervisor". L1 is a +guest virtual machine that is directly run under L0 and is initiated +and controlled by L0. L2 is a guest virtual machine that is initiated +and controlled by L1 acting as a hypervisor. A significant design change +wrt existing API is that now the entire L2 state is maintained within L0. + +Existing Nested-HV API +====================== + +Linux/KVM has had support for Nesting as an L0 or L1 since 2018 + +The L0 code was added:: + + commit 8e3f5fc1045dc49fd175b978c5457f5f51e7a2ce + Author: Paul Mackerras <paulus@ozlabs.org> + Date: Mon Oct 8 16:31:03 2018 +1100 + KVM: PPC: Book3S HV: Framework and hcall stubs for nested virtualization + +The L1 code was added:: + + commit 360cae313702cdd0b90f82c261a8302fecef030a + Author: Paul Mackerras <paulus@ozlabs.org> + Date: Mon Oct 8 16:31:04 2018 +1100 + KVM: PPC: Book3S HV: Nested guest entry via hypercall + +This API works primarily using a signal hcall h_enter_nested(). This +call made by the L1 to tell the L0 to start an L2 vCPU with the given +state. The L0 then starts this L2 and runs until an L2 exit condition +is reached. Once the L2 exits, the state of the L2 is given back to +the L1 by the L0. The full L2 vCPU state is always transferred from +and to L1 when the L2 is run. The L0 doesn't keep any state on the L2 +vCPU (except in the short sequence in the L0 on L1 -> L2 entry and L2 +-> L1 exit). + +The only state kept by the L0 is the partition table. The L1 registers +it's partition table using the h_set_partition_table() hcall. All +other state held by the L0 about the L2s is cached state (such as +shadow page tables). + +The L1 may run any L2 or vCPU without first informing the L0. It +simply starts the vCPU using h_enter_nested(). The creation of L2s and +vCPUs is done implicitly whenever h_enter_nested() is called. + +In this document, we call this existing API the v1 API. + +New PAPR API +=============== + +The new PAPR API changes from the v1 API such that the creating L2 and +associated vCPUs is explicit. In this document, we call this the v2 +API. + +h_enter_nested() is replaced with H_GUEST_VCPU_RUN(). Before this can +be called the L1 must explicitly create the L2 using h_guest_create() +and any associated vCPUs() created with h_guest_create_vCPU(). Getting +and setting vCPU state can also be performed using h_guest_{g|s}et +hcall. + +The basic execution flow is for an L1 to create an L2, run it, and +delete it is: + +- L1 and L0 negotiate capabilities with H_GUEST_{G,S}ET_CAPABILITIES() + (normally at L1 boot time). + +- L1 requests the L0 to create an L2 with H_GUEST_CREATE() and receives a token + +- L1 requests the L0 to create an L2 vCPU with H_GUEST_CREATE_VCPU() + +- L1 and L0 communicate the vCPU state using the H_GUEST_{G,S}ET() hcall + +- L1 requests the L0 to run the vCPU using H_GUEST_RUN_VCPU() hcall + +- L1 deletes L2 with H_GUEST_DELETE() + +For more details, please refer: + +[1] Linux Kernel documentation (upstream documentation commit): + +commit 476652297f94a2e5e5ef29e734b0da37ade94110 +Author: Michael Neuling <mikey@neuling.org> +Date: Thu Sep 14 13:06:00 2023 +1000 + + docs: powerpc: Document nested KVM on POWER + + Document support for nested KVM on POWER using the existing API as well + as the new PAPR API. This includes the new HCALL interface and how it + used by KVM. + + Signed-off-by: Michael Neuling <mikey@neuling.org> + Signed-off-by: Jordan Niethe <jniethe5@gmail.com> + Signed-off-by: Michael Ellerman <mpe@ellerman.id.au> + Link: https://msgid.link/20230914030600.16993-12-jniethe5@gmail.com |