On Tue, Sep 22, 2026 at 07:52:25PM +0530, Aneesh Kumar K.V wrote: > "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.
Ack, understood. Feel free to cc- me on that when you send it out! > > I also want to point out that there are use cases for pre-faulting while > the VM is running. Ack yeah understood! And it does support that fine :) I added further reasoning about pKVM in the cover/commit msg in the respin ([0]) so hopefully that helps clarify from that sid eof things. > > > > > 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. Ack yeah, then the series should hopefully work for you. If you could test it and report any issues I'd be grateful! Thanks :) > > > > > 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. Thanks! > > > (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. OK cool, let me know if you hit any issues. > > -aneesh -- Cheers, Lorenzo [0]: https://lore.kernel.org/kvmarm/[email protected]/T/#t
