"Lorenzo Stoakes (ARM)" <[email protected]> writes: > On Tue, Sep 22, 2026 at 03:28:33PM +0530, Aneesh Kumar K.V wrote: >> > This is because pKVM instantiates vCPUs upon run, >> > >> >> Can pKVM instantiate the hyp vCPU during pre-faulting ? > > It would be an unusual and unexpected thing to do - suddenly a pre-fault > operation is initialising a vCPU explicitly for pKVM. > > A caller is not going to reasonably expect this and might treat a failure to > pre-alloc as fine to carry on whereas in fact it was a failure to initailised > a > pKVM vCPU. > > It'd also require significant changes to how pKVM is set up, right now it's > hardcoded to be done unconditionally at run via kvm_arch_vcpu_run_pid_change() > -> pkvm_create_hyp_vcpu(), so all that would have to change and be checked and > tested and... that'd be really out of scope I think :) > > And pre-faulting really makes most sense BEFORE you run a VM. It doesn't make > so > much sense mid-run. > > But more fundamentally, the stage 2 page tables, as I understand it, are owned > by pKVM and so aren't really available to be pre-faulted. > > Maybe unprotected-under-pKVM VMs but then it's questionable as to how useful > that would be given that it would be confusing to users vs. how it works for > other VMs. > > So in general, no I don't think it's a good idea. > > And even if we wanted to pursue some version of this, it's _definitely_ out > of scope for the initial pre-faulting bring-up series. > >> >> >> > 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 > > You mean you'd prefer to use? Or you are using it on another arch? > >> with CoCo guest. This ensures that a trusted device can DMA to private >> memory before the guest accesses it. > > Hm what do you mean by private memory? > > I see: > > #ifndef CONFIG_KVM_GENERIC_MEMORY_ATTRIBUTES > static inline bool kvm_arch_has_private_mem(struct kvm *kvm) > { > return false; > } > #endif > > And only x86 selects KVM_GENERIC_MEMORY_ATTRIBUTES? > > Do you mean something else? >
I am using this with ARM CCA-DA, based on the patch series from Jack Thomson <[email protected]>. https://gitlab.arm.com/linux-arm/kvmtool-cca/-/commit/80e7aad61c5639de2f0cb4a5525dad0c96156428 We do this while the VM is running. -aneesh
