Re: [PATCH 5/6] KVM: nSVM: avoid loading PDPTRs after migration when possible

[Sean Christopherson <seanjc@google.com>]

On Thu, Apr 01, 2021, Maxim Levitsky wrote:
> if new KVM_*_SREGS2 ioctls are used, the PDPTRs are
> part of the migration state and thus are loaded
> by those ioctls.
> 
> Signed-off-by: Maxim Levitsky <mlevitsk@redhat.com>
> ---
>  arch/x86/kvm/svm/nested.c | 15 +++++++++++++--
>  1 file changed, 13 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/x86/kvm/svm/nested.c b/arch/x86/kvm/svm/nested.c
> index ac5e3e17bda4..b94916548cfa 100644
> --- a/arch/x86/kvm/svm/nested.c
> +++ b/arch/x86/kvm/svm/nested.c
> @@ -373,10 +373,9 @@ static int nested_svm_load_cr3(struct kvm_vcpu *vcpu, unsigned long cr3,
>  		return -EINVAL;
>  
>  	if (!nested_npt && is_pae_paging(vcpu) &&
> -	    (cr3 != kvm_read_cr3(vcpu) || pdptrs_changed(vcpu))) {
> +	    (cr3 != kvm_read_cr3(vcpu) || !kvm_register_is_available(vcpu, VCPU_EXREG_PDPTR)))
>  		if (CC(!load_pdptrs(vcpu, vcpu->arch.walk_mmu, cr3)))

What if we ditch the optimizations[*] altogether and just do:

	if (!nested_npt && is_pae_paging(vcpu) &&
	    CC(!load_pdptrs(vcpu, vcpu->arch.walk_mmu, cr3))
		return -EINVAL;

Won't that obviate the need for KVM_{GET|SET}_SREGS2 since KVM will always load
the PDPTRs from memory?  IMO, nested migration with shadowing paging doesn't
warrant this level of optimization complexity.

[*] For some definitions of "optimization", since the extra pdptrs_changed()
    check in the existing code is likely a net negative.

>  			return -EINVAL;
> -	}
>  
>  	/*
>  	 * TODO: optimize unconditional TLB flush/MMU sync here and in