Re: [PATCH v2 2/5] mmu: don't set the present bit unconditionally

Paolo Bonzini
2016-07-13T08:10:34Z

On 13/07/2016 00:18, Bandan Das wrote:
> + /*
> + * In the non-EPT case, execonly is not valid and so
> + * the following line is equivalent to spte |= PT_PRESENT_MASK.

I think the comment need not mention non-EPT.

> + * For the EPT case, shadow_present_mask is 0 if hardware
> + * supports it and we honor whatever way the guest set it.

if hardware supports exec-only page table entries.

> + * See: FNAME(gpte_access) in paging_tmpl.h
> + */
> + spte |= shadow_present_mask;
> if (!speculative)
> spte |= shadow_accessed_mask;
>
> diff --git a/arch/x86/kvm/paging_tmpl.h b/arch/x86/kvm/paging_tmpl.h
> index bc019f7..f2741db 100644
> --- a/arch/x86/kvm/paging_tmpl.h
> +++ b/arch/x86/kvm/paging_tmpl.h
> @@ -181,13 +181,19 @@ no_present:
> return true;
> }
>
> +/*
> + * For PTTYPE_EPT, a page table can be executable but not readable
> + * on supported processors. Therefore, set_spte does not automatically
> + * set bit 0 if execute only is supported. Here, we repurpose ACC_USER_MASK
> + * to signify readability since it isn't used in the EPT case
> + */
> static inline unsigned FNAME(gpte_access)(struct kvm_vcpu *vcpu, u64 gpte)
> {
> unsigned access;
> #if PTTYPE == PTTYPE_EPT
> access = ((gpte & VMX_EPT_WRITABLE_MASK) ? ACC_WRITE_MASK : 0) |
> ((gpte & VMX_EPT_EXECUTABLE_MASK) ? ACC_EXEC_MASK : 0) |
> - ACC_USER_MASK;
> + ((gpte & VMX_EPT_READABLE_MASK) ? ACC_USER_MASK : 0);
> #else
> BUILD_BUG_ON(ACC_EXEC_MASK != PT_PRESENT_MASK);
> BUILD_BUG_ON(ACC_EXEC_MASK != 1);
> diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
> index 64a79f2..f73b5dc 100644
> --- a/arch/x86/kvm/vmx.c
> +++ b/arch/x86/kvm/vmx.c
> @@ -6366,10 +6366,13 @@ static __init int hardware_setup(void)
> vmx_disable_intercept_msr_write_x2apic(0x83f);
>
> if (enable_ept) {
> - kvm_mmu_set_mask_ptes(0ull,
> + kvm_mmu_set_mask_ptes(VMX_EPT_READABLE_MASK,
> (enable_ept_ad_bits) ? VMX_EPT_ACCESS_BIT : 0ull,
> (enable_ept_ad_bits) ? VMX_EPT_DIRTY_BIT : 0ull,
> - 0ull, VMX_EPT_EXECUTABLE_MASK);
> + 0ull, VMX_EPT_EXECUTABLE_MASK,
> + cpu_has_vmx_ept_execute_only() ?
> + 0ull : PT_PRESENT_MASK);

Better to use VMX_EPT_READABLE_MASK here, which removes the need for the
BUILD_BUG_ON.

I can do these changes myself.

Paolo

> + BUILD_BUG_ON(PT_PRESENT_MASK != VMX_EPT_READABLE_MASK);