Re: [PATCH] KVM: x86: Allow guest to set EFER.NX=1 on non-PAE 32-bit kernels

[Paolo Bonzini]

On 05/08/21 20:38, Sean Christopherson wrote:
> Fast forward to today, and KVM has long since stopped running the guest
> with the host's EFER.NX.  Not only does KVM context switch EFER if
> host.EFER.NX=1 && guest.EFER.NX=0, KVM also forces host.EFER.NX=0 &&
> guest.EFER.NX=1 when using shadow paging (to emulate SMEP).  Furthermore,
> the entire motivation for the restriction was made obsolete over a decade
> ago when Intel added dedicated host and guest EFER fields in the VMCS
> (Nehalem timeframe), which reduced the overhead of context switching EFER
> from 400+ cycles (2 * WRMSR + 1 * RDMSR) to a mere ~2 cycles.

Both pretty good points. :)  Queued, thanks.

Paolo