"Lorenzo Stoakes (ARM)" <[email protected]> writes: > On Tue, Sep 22, 2026 at 06:37:04PM +0530, Aneesh Kumar K.V wrote: >> "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. > > Right, that's a non-mainline kernel I guess? Which presumably implements > private memory. > > Jack himself experienced a panic with his pKVM code, so the code you're using > is > not upstreamable, unfortunately. And he'd already shelved pKVM support AFAICT. > > And reviewers pointed out actually implementing the pKVM stuff properly would > be > quite involved, even if you wanted to do that (hence follow-up). > > Also you end up stuck with the same problems as I mentioned above - you can't > sanely bring the vCPU pre-run, so now you have extremely weird behaviour - > only > pre-faults if vCPU initialised, running, and unprotected pKVM. >
IIUC, pkvm_pgtable_stage2_map() only uses the hyp vCPU's pKVM memcache (&vcpu->vcpu.arch.pkvm_memcache). I agree that this does not need to be addressed in this series. However, there is also a desire to keep the CCA and pKVM code paths similar by using helpers such as kvm_vm_is_protected(). Since an RMM can create a stage-2 mapping without a REC (vCPU), rejecting pre-faulting for all protected VM configurations may be incorrect. I can handle this in my series though. I also want to point out that there are use cases for pre-faulting while the VM is running. > > Protected pKVM support is a whole other layer of complexity and it's not > obvious > that you're really achieving what pre-fault is supposed to. > > In any case Oliver literally just asked me to _simplify_ weird edge cases for > this series :) so I am not sure something like that is going to be accepted. > > Are you sure you're actually running in pKVM mode btw? CCA doesn't AFAICT? In > which case this series _should_ work fine for you. > It is not pKVM; it runs in Realm mode. > > Anyway, if we really do need to add something for pKVM it needs to be a follow > up. Let's get the basics working first :) > sure. > (Note that kvmtool will need to be updated to retry pre-fault on -EAGAIN, > -EINTR > as this series can, albeit unlikely, return -EAGAIN.) > I will check this when I rebase my kernel onto this series. -aneesh
