Re: [PATCH v2 4/5] mmu: change unconditional setting of the u bit in fault bitmap

Paolo Bonzini
2016-07-13T08:24:05Z

On 13/07/2016 00:18, Bandan Das wrote:
> For the nested EPT case, we assume that the read bit (u) is
> always set since we used to unconditionally set it in set_spte().
> Modify it to only be set when host ept execute only support
> isn't present.
>
> Signed-off-by: Bandan Das <bsd@redhat.com>
> ---
> arch/x86/kvm/mmu.c | 7 ++++---
> 1 file changed, 4 insertions(+), 3 deletions(-)
>
> diff --git a/arch/x86/kvm/mmu.c b/arch/x86/kvm/mmu.c
> index c364dcb..566eea5 100644
> --- a/arch/x86/kvm/mmu.c
> +++ b/arch/x86/kvm/mmu.c
> @@ -3923,9 +3923,10 @@ static void update_permission_bitmask(struct kvm_vcpu *vcpu,
> * clearer.
> */
> smap = cr4_smap && u && !uf && !ff;
> - } else
> - /* Not really needed: no U/S accesses on ept */
> - u = 1;
> + } else {
> + if (shadow_present_mask)
> + u = 1;
> + }
>
> fault = (ff && !x) || (uf && !u) || (wf && !w) ||
> (smapf && smap);
>

This is needed too, in order to look up with the correct uf:

diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
index 003618e324ce..941d345ebd2c 100644
--- a/arch/x86/kvm/vmx.c
+++ b/arch/x86/kvm/vmx.c
@@ -6014,12 +6014,14 @@ 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;

vcpu->arch.exit_qualification = exit_qualification;

Otherwise you always lookup the permission bitmap for an uf=0 case.
This was the meaning of the "Not really needed" comment.

Paolo