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    SubjectRe: [PATCH v2 0/5] Add support for EPT execute only for nested hypervisors
    Date
    Paolo Bonzini <pbonzini@redhat.com> writes:

    > On 13/07/2016 00:18, Bandan Das wrote:
    >> v1 of this series posted at https://lkml.org/lkml/2016/6/28/7
    >>
    >> Changes since v1:
    >> - 1/5 : modify is_shadow_present_pte to check against 0xffffffff
    >> Reasoning provided in commit message.
    >> - 2/5 : Removed 2/5 from v1 since kvm doesn't use execute only.
    >> 3/5 from v1 is now 2/5. Introduce shadow_present_mask that
    >> signifies whether ept execute only is supported. Add/remove some
    >> comments as suggested in v1.
    >> - 3/5 : 4/5 from v1 is now 3/5.
    >> - 4/5 : update_permission_bitmask now sets u=1 only if host doesn't
    >> support ept execute only.
    >> - 5/5 : No change
    >
    > These are the diffs I have after review, do they look okay?
    >
    > diff --git a/arch/x86/kvm/mmu.c b/arch/x86/kvm/mmu.c
    > index 190c0559c221..bd2535fdb9eb 100644
    > --- a/arch/x86/kvm/mmu.c
    > +++ b/arch/x86/kvm/mmu.c
    > @@ -2524,11 +2524,10 @@ static int set_spte(struct kvm_vcpu *vcpu, u64 *sptep,
    > return 0;
    >
    > /*
    > - * In the non-EPT case, execonly is not valid and so
    > - * the following line is equivalent to spte |= PT_PRESENT_MASK.
    > * For the EPT case, shadow_present_mask is 0 if hardware
    > - * supports it and we honor whatever way the guest set it.
    > - * See: FNAME(gpte_access) in paging_tmpl.h
    > + * supports exec-only page table entries. In that case,
    > + * ACC_USER_MASK and shadow_user_mask are used to represent
    > + * read access. See FNAME(gpte_access) in paging_tmpl.h.
    > */

    I would still prefer a note about the non-EPT case, makes it easy to
    understand.

    > spte |= shadow_present_mask;
    > if (!speculative)
    > @@ -3923,9 +3922,6 @@ static void update_permission_bitmask(struct kvm_vcpu *vcpu,
    > * clearer.
    > */
    > smap = cr4_smap && u && !uf && !ff;
    > - } else {
    > - if (shadow_present_mask)
    > - u = 1;
    > }
    >
    > fault = (ff && !x) || (uf && !u) || (wf && !w) ||
    > diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
    > index 576c47cda1a3..dfef081e76c0 100644
    > --- a/arch/x86/kvm/vmx.c
    > +++ b/arch/x86/kvm/vmx.c
    > @@ -6120,12 +6120,14 @@ static int handle_ept_violation(struct kvm_vcpu *vcpu)
    > gpa = vmcs_read64(GUEST_PHYSICAL_ADDRESS);
    > trace_kvm_page_fault(gpa, exit_qualification);
    >
    > - /* It is a write fault? */
    > + /* it is a read fault? */
    > + error_code = (exit_qualification << 2) & PFERR_USER_MASK;
    > + /* it is a write fault? */
    > error_code = exit_qualification & PFERR_WRITE_MASK;
    > /* It is a fetch fault? */
    > error_code |= (exit_qualification << 2) & PFERR_FETCH_MASK;
    > /* ept page table is present? */
    > - error_code |= (exit_qualification >> 3) & PFERR_PRESENT_MASK;
    > + error_code |= (exit_qualification & 0x38) != 0;
    >

    Thank you for the thorough review here. I missed that we didn't set the read bit
    at all. I am still a little unclear how permission_fault works though...


    > vcpu->arch.exit_qualification = exit_qualification;
    >
    > @@ -6474,8 +6476,7 @@ static __init int hardware_setup(void)
    > (enable_ept_ad_bits) ? VMX_EPT_DIRTY_BIT : 0ull,
    > 0ull, VMX_EPT_EXECUTABLE_MASK,
    > cpu_has_vmx_ept_execute_only() ?
    > - 0ull : PT_PRESENT_MASK);
    > - BUILD_BUG_ON(PT_PRESENT_MASK != VMX_EPT_READABLE_MASK);
    > + 0ull : VMX_EPT_READABLE_MASK);

    I wanted to keep it the former way because "PT_PRESENT_MASK is equal to VMX_EPT_READABLE_MASK"
    is an assumption all throughout. I wanted to use this section to catch mismatches.

    Bandan

    > ept_set_mmio_spte_mask();
    > kvm_enable_tdp();
    > } else

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