Re: [RFC] KVM: x86/mmu: Prefetch forward run of pages on TDP page faults

[Sean Christopherson]

On Tue, Aug 25, 2026, Marco Marangoni wrote:
> It's worth mentioning that I also evaluated using the existing
> KVM_PRE_FAULT_MEMORY ioctl, but this doesn't work well for our use-case, as
> it requires the vCPU to be paused.

What about if/when KVM Userfault[*] comes along?  I.e. pre-fault memory when the
vCPU exits to userspace.

If KVM Userfault + KVM_PRE_FAULT_MEMORY is viable for Firecracker and provides
the desired performance, I would strongly prefer that approach over KVM doing its
own prefetching.  That way other architectures don't need to implement prefetching
as well (arm64 already supports KVM_PRE_FAULT_MEMORY, and RISC-V has patches posted).

I'm aiming to get back to that series "soon", this would simply be more motivation
to get KVM Userfault landed.

[*] https://lore.kernel.org/all/20250618042424.330664-1-jthoughton@xxxxxxxxxx