Re: [PATCH v2 00/13] KVM: arm64: Add KVM_PRE_FAULT_MEMORY support

[Aneesh Kumar K.V <aneesh.kumar@kernel.org>]

"Lorenzo Stoakes (ARM)" <ljs@kernel.org> 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 <jackabt.amazon@gmail.com>.

https://gitlab.arm.com/linux-arm/kvmtool-cca/-/commit/80e7aad61c5639de2f0cb4a5525dad0c96156428

We do this while the VM is running.

-aneesh