Re: [RFC PATCH 1/6] KVM: X86: Check root_level only in fast_pgd_switch()

[Sean Christopherson <seanjc@google.com>]

On Fri, Dec 10, 2021, Lai Jiangshan wrote:
> From: Lai Jiangshan <laijs@linux.alibaba.com>
> 
> If root_level >= 4, shadow_root_level must be >= 4 too.

Please explain why so that it's explicitly clear why this ok, e.g.

  Drop the shadow_root_level check when determining if a "fast" PGD switch
  is allowed, as KVM never shadows 64-bit guest pages tables with PAE paging
  (32-bit KVM doesn't support 64-bit guests).

with that:

Reviewed-by: Sean Christopherson <seanjc@google.com>

> Checking only root_level can reduce a check.
>
> Signed-off-by: Lai Jiangshan <laijs@linux.alibaba.com>
> ---
>  arch/x86/kvm/mmu/mmu.c | 3 +--
>  1 file changed, 1 insertion(+), 2 deletions(-)
> 
> diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
> index 11b06d536cc9..846a2e426e0b 100644
> --- a/arch/x86/kvm/mmu/mmu.c
> +++ b/arch/x86/kvm/mmu/mmu.c
> @@ -4136,8 +4136,7 @@ static bool fast_pgd_switch(struct kvm_vcpu *vcpu, gpa_t new_pgd,
>  	 * having to deal with PDPTEs. We may add support for 32-bit hosts/VMs
>  	 * later if necessary.
>  	 */
> -	if (mmu->shadow_root_level >= PT64_ROOT_4LEVEL &&
> -	    mmu->root_level >= PT64_ROOT_4LEVEL)
> +	if (mmu->root_level >= PT64_ROOT_4LEVEL)
>  		return cached_root_available(vcpu, new_pgd, new_role);
>  
>  	return false;
> -- 
> 2.19.1.6.gb485710b
>