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

Paolo Bonzini
2016-07-13T09:07:20Z

On 13/07/2016 10:23, Paolo Bonzini wrote:
>
>
> 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;

... in fact I think this should be removed completely.

If shadow_present_mask is set, the R bit will always be set in a present
PTE and FNAME(gpte_access) will change that to ACC_USER_MASK---which in
turn means that u=1.

Paolo

>> + }
>>
>> 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
>