Re: [RFC PATCH 2/4] mmu: Update ept specific valid bit values

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

On 06/21/2016 11:12 AM, Bandan Das wrote:
> Until now , is_present_gpte checks for all bits set (XWR) and
> is_shadow_present_pte() checks for the present bit set. To support
> execute only mappings we should teach these functions to distinguish 100
> as valid based on host support.
>
> Signed-off-by: Bandan Das <bsd@redhat.com>
> ---
>   arch/x86/kvm/mmu.c         | 5 ++++-
>   arch/x86/kvm/paging_tmpl.h | 7 ++++++-
>   2 files changed, 10 insertions(+), 2 deletions(-)
>
> diff --git a/arch/x86/kvm/mmu.c b/arch/x86/kvm/mmu.c
> index 37b01b1..57d8696 100644
> --- a/arch/x86/kvm/mmu.c
> +++ b/arch/x86/kvm/mmu.c
> @@ -308,7 +308,10 @@ static int is_nx(struct kvm_vcpu *vcpu)
>
>   static int is_shadow_present_pte(u64 pte)
>   {
> -	return pte & PT_PRESENT_MASK && !is_mmio_spte(pte);
> +	int xbit = shadow_xonly_valid ? pte & shadow_x_mask : 0;
> +
> +	return (pte & PT_PRESENT_MASK) | xbit
> +		&& !is_mmio_spte(pte);

It could be simply use pte & (PT_PRESENT_MASK | shadow_x_mask) instead,
as this is host controlled page table and we never clear single
PT_PRESENT_MASK bit (we only fully clear low 32 bit).

>   }
>
>   static int is_large_pte(u64 pte)
> diff --git a/arch/x86/kvm/paging_tmpl.h b/arch/x86/kvm/paging_tmpl.h
> index bc019f7..9f5bd06 100644
> --- a/arch/x86/kvm/paging_tmpl.h
> +++ b/arch/x86/kvm/paging_tmpl.h
> @@ -133,7 +133,12 @@ static inline int FNAME(is_present_gpte)(unsigned long pte)
>   #if PTTYPE != PTTYPE_EPT
>   	return is_present_gpte(pte);
>   #else
> -	return pte & 7;
> +	/*
> +	 * For EPT, bits [2:0] can be 001, 100 or 111
> +	 * Further, for 100, logical processor should support
> +	 * execute-only.
> +	 */
> +	return (pte & 1) || (shadow_xonly_valid && (pte & 4));

No.

For !shadow_xonly_valid guest, 100b on gpte can not pass FNAME(is_present_gpte)
so that we can get a error code without RSVD, finally a EPT-violation is injected
rather than EPT-misconfig.

Actually, current code handles shadow_xonly_valid very well, please refer to
kvm_init_shadow_ept_mmu(), there already has handled the case of 'execonly'.