"Lorenzo Stoakes (ARM)" <[email protected]> writes: > This series implements the KVM stage 2 page table pre-faulting feature for > arm64. >
> == Foundations == > > The series begins by establishing the required foundations: > > 1. Update kvm_s2_fault_desc to store the exception syndrome register (ESR) > value independently, and update all code paths to use this value > exclusively. > > This is required to generate a synthetic fault for pre-faulting without > inadvertently obtaining an incorrect ESR from elsewhere. > > 2. Update kvm_s2_fault_desc to store the kvm_s2_mmu independently, and > update all code paths to use this value exclusively. > > This is similarly required to generate a synthetic fault against the > canonical stage-2 MMU. It would make no sense for pre-faulting to modify > nested shadow page tables, so the code must not obtain an incorrect MMU > from elsewhere. > > 3. Update the abort paths that consume kvm_s2_fault_desc to also return a > kvm_s2_fault_result. > > Pre-faulting needs to know the granule size handled by the fault, so the > abort paths must return that information. > > 4. Pass walk flags to kvm_pgtable_get_leaf() to allow page-table walks > under the MMU read lock. > > This is Jack's patch verbatim. It allows KVM_PGTABLE_WALK_SHARED to be used > when walking page tables under the MMU read lock. > > The API permits pre-faulting to run in parallel, and the read lock prevents > the page tables from being torn down during the walk. > > == Implementation == > > Pre-faulting is implemented in kvm_arch_vcpu_pre_fault_memory(), which > pre-faults the stage-2 page tables for a specific GPA (the guest IPA on > arm64). > > kvm_vcpu_pre_fault_memory() calls this function for each GPA in the range > requested by userspace through the KVM_PRE_FAULT_MEMORY ioctl. > > The implementation is straightforward: walk the stage-2 page tables for > the GPA and, if it is unmapped, populate the mapping by handling a > synthetic page fault. [ ... 47 lines skipped ... ] > > pKVM is not supported regardless of whether the VM is protected or > not. > > This is because pKVM instantiates vCPUs upon run, > Can pKVM instantiate the hyp vCPU during pre-faulting ? > but pre-faulting is typically performed before a vCPU is run. It would be > confusing and > inconsistent to error out on non-running vCPUs but to pre-fault running > ones. I use KVM pre-faulting when transitioning pages from shared to private with CoCo guest. This ensures that a trusted device can DMA to private memory before the guest accesses it. -aneesh
