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

[James Houghton]

On Tue, Aug 25, 2026 at 8:00 AM Sean Christopherson <seanjc@xxxxxxxxxx> wrote:
>
> 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.

My understanding is that Firecracker performs its userfaultfd
operations in a separate process. KVM Userfault doesn't break this, it
just means Firecracker would need a way to signal the other process
when we get KVM_EXIT_USERFAULTs. Firecracker might be using async PF;
using KVM Userfault basically disables async PF.

> 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 also really like this approach if it is workable for Firecracker.

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

Yay!

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