Re: [PATCH 0/6] KVM ARM64 pre_fault_memory

Oliver Upton
2025-09-11T18:57:46Z

Hi Jack,

On Thu, Sep 11, 2025 at 02:46:42PM +0100, Jack Thomson wrote:
> From: Jack Thomson <jackabt@amazon.com>
>
> Overview:
>
> This patch series adds ARM64 support for the KVM_PRE_FAULT_MEMORY
> feature, which was previously only available on x86 [1]. This allows
> a reduction in the number of stage-2 faults during execution. This is
> beneficial in post-copy migration scenarios, particularly in memory
> intensive applications, where high latencies are experienced due to
> the stage-2 faults when pre-populating memory via UFFD / memcpy.

Thanks for posting the series. More of a general comment on the UAPI
documentation:

"... However, KVM does not mark any newly created stage-2 PTE as Accessed."

This behavior is x86-specific since kvm_pgtable_stage2_map() will lay
down PTEs with the AF set. Probably shouldn't have documented the
internal state of the stage-2 in the first place but oh well, please
just update the UAPI description to make it clear this is x86-specific.

Thanks
Oliver