Re: [RFC PATCH 20/21] KVM: x86: Force a prefetch fault's max mapping level to 4KB for TDX

[Binbin Wu <binbin.wu@linux.intel.com>]

On 4/24/2025 11:09 AM, Yan Zhao wrote:
> Introduce a "prefetch" parameter to the private_max_mapping_level hook and
> enforce the max mapping level of a prefetch fault for private memory to be
> 4KB. This is a preparation to enable the ignoring huge page splitting in
> the fault path.
>
> If a prefetch fault results in a 2MB huge leaf in the mirror page table,
> there may not be a vCPU available to accept the corresponding 2MB huge leaf
> in the S-EPT if the TD is not configured to receive #VE for page
> acceptance. Consequently, if a vCPU accepts the page at 4KB level, it will
> trigger an EPT violation to split the 2MB huge leaf generated by the
> prefetch fault.
>
> Since handling the BUSY error from SEAMCALLs for huge page splitting is
> more comprehensive in the fault path, which is with kvm->mmu_lock held for
> reading, force the max mapping level of a prefetch fault of private memory
> to be 4KB to prevent potential splitting.
>
> Since prefetch faults for private memory are uncommon after the TD's build
> time, enforcing a 4KB mapping level is unlikely to cause any performance
> degradation.
I am wondering what are the use cases for KVM_PRE_FAULT_MEMORY.
Is there an API usage guide to limit that userspace shouldn't use it for a large
amount of memory pre-fault? If no, and userspace uses it to pre-fault a lot of
memory, this "unlikely to cause any performance degradation" might be not true.