Re: [PATCH v4 9/9] KVM: x86: LAM: Expose LAM CPUID to user space VMM

Yu Zhang
2023-02-21T11:13:40Z

>
> The special handling for LA57 is from the patch "kvm: x86: Return LA57
> feature based on hardware capability".
> https://lore.kernel.org/lkml/1548950983-18458-1-git-send-email-yu.c.zhang@linux.intel.com/
>
> The reason is host kernel may disable 5-level paging using cmdline parameter
> 'no5lvl', and it will clear the feature bit for LA57 in boot_cpu_data.
> boot_cpu_data is queried in kvm_set_cpu_caps to derive kvm cpu cap masks.
>
> " VMs can still benefit from extended linear address width, e.g. to enhance
> features like ASLR" even when host  doesn't use 5-level paging.
> So, the patch sets LA57 based on hardware capability.
>
> I was just wondering  whether LAM could be the similar case that the host
> disabled the feature somehow (e.g via clearcpuid), and the guest still want
> to use it.

Paging modes in root & non-root are orthogonal, so should LAM.

B.R.
Yu