Re: [RFC PATCH 3/4] mmu: don't set the present bit unconditionally

[Xiao Guangrong <guangrong.xiao@linux.intel.com>]

On 06/21/2016 11:12 AM, Bandan Das wrote:
> To support execute only mappings on behalf of L1 hypervisors,
> we teach set_spte to honor L1's valid XWR bits. This is only
> if host supports EPT execute only. Use ACC_USER_MASK to signify
> if the L1 hypervisor has the present bit set.
>
> Signed-off-by: Bandan Das <bsd@redhat.com>
> ---
>   arch/x86/kvm/mmu.c         | 11 ++++++++---
>   arch/x86/kvm/paging_tmpl.h |  2 +-
>   2 files changed, 9 insertions(+), 4 deletions(-)
>
> diff --git a/arch/x86/kvm/mmu.c b/arch/x86/kvm/mmu.c
> index 57d8696..3ca1a99 100644
> --- a/arch/x86/kvm/mmu.c
> +++ b/arch/x86/kvm/mmu.c
> @@ -2528,7 +2528,8 @@ static int set_spte(struct kvm_vcpu *vcpu, u64 *sptep,
>   	if (set_mmio_spte(vcpu, sptep, gfn, pfn, pte_access))
>   		return 0;
>
> -	spte = PT_PRESENT_MASK;
> +	if (!shadow_xonly_valid)
> +		spte = PT_PRESENT_MASK;

The xonly info can be fetched from vcpu->mmu. shadow_xonly_valid looks like
can be dropped.

>   	if (!speculative)
>   		spte |= shadow_accessed_mask;
>
> @@ -2537,8 +2538,12 @@ static int set_spte(struct kvm_vcpu *vcpu, u64 *sptep,
>   	else
>   		spte |= shadow_nx_mask;
>
> -	if (pte_access & ACC_USER_MASK)
> -		spte |= shadow_user_mask;
> +	if (pte_access & ACC_USER_MASK) {
> +		if (shadow_xonly_valid)
> +			spte |= PT_PRESENT_MASK;
> +		else
> +			spte |= shadow_user_mask;
> +	}

It can be simplified by setting shadow_user_mask to PT_PRESENT_MASK
if ept enabled.