Hi Marc,
On Wed, Dec 11, 2019 at 04:56:48PM +0000, Marc Zyngier wrote:
> A device mapping is normally always mapped at Stage-2, since there
> is very little gain in having it faulted in.
It is actually becoming less clear to me what the real benefits of
pre-populating the stage 2 page table are, especially given that we can
provoke a situation where they're faulted in anyhow. Do you recall if
we had any specific case that motivated us to pre-fault in the pages?
>
> Nonetheless, it is possible to end-up in a situation where the device
> mapping has been removed from Stage-2 (userspace munmaped the VFIO
> region, and the MMU notifier did its job), but present in a userspace
> mapping (userpace has mapped it back at the same address). In such
> a situation, the device mapping will be demand-paged as the guest
> performs memory accesses.
>
> This requires to be careful when dealing with mapping size, cache
> management, and to handle potential execution of a device mapping.
>
> Cc: stable@xxxxxxxxxxxxxxx
> Reported-by: Alexandru Elisei <alexandru.elisei@xxxxxxx>
> Signed-off-by: Marc Zyngier <maz@xxxxxxxxxx>
> ---
> virt/kvm/arm/mmu.c | 21 +++++++++++++++++----
> 1 file changed, 17 insertions(+), 4 deletions(-)
>
> diff --git a/virt/kvm/arm/mmu.c b/virt/kvm/arm/mmu.c
> index a48994af70b8..0b32a904a1bb 100644
> --- a/virt/kvm/arm/mmu.c
> +++ b/virt/kvm/arm/mmu.c
> @@ -38,6 +38,11 @@ static unsigned long io_map_base;
> #define KVM_S2PTE_FLAG_IS_IOMAP (1UL << 0)
> #define KVM_S2_FLAG_LOGGING_ACTIVE (1UL << 1)
>
> +static bool is_iomap(unsigned long flags)
> +{
> + return flags & KVM_S2PTE_FLAG_IS_IOMAP;
> +}
> +
nit: I'm not really sure this indirection makes the code more readable,
but I guess that's a matter of taste.
> static bool memslot_is_logging(struct kvm_memory_slot *memslot)
> {
> return memslot->dirty_bitmap && !(memslot->flags & KVM_MEM_READONLY);
> @@ -1698,6 +1703,7 @@ static int user_mem_abort(struct kvm_vcpu *vcpu, phys_addr_t fault_ipa,
>
> vma_pagesize = vma_kernel_pagesize(vma);
> if (logging_active ||
> + (vma->vm_flags & VM_PFNMAP) ||
WHat is actually the rationale for this?
Why is a huge mapping not permitted to device memory?
Are we guaranteed that VM_PFNMAP on the vma results in device mappings?
I'm not convinced this is the case, and it would be better if we can
stick to a single primitive (either kvm_is_device_pfn, or VM_PFNMAP) to
detect device mappings.
As a subsequent patch, I'd like to make sure that at the very least our
memslot prepare function follows the exact same logic for mapping device
memory as a fault-in approach does, or that we simply always fault pages
in.
> !fault_supports_stage2_huge_mapping(memslot, hva, vma_pagesize)) {
> force_pte = true;
> vma_pagesize = PAGE_SIZE;
> @@ -1760,6 +1766,9 @@ static int user_mem_abort(struct kvm_vcpu *vcpu, phys_addr_t fault_ipa,
> writable = false;
> }
>
> + if (exec_fault && is_iomap(flags))
> + return -ENOEXEC;
> +
nit: why don't you just do this when checking kvm_is_device_pfn() and
avoid having logic in two places to deal with this case?
> spin_lock(&kvm->mmu_lock);
> if (mmu_notifier_retry(kvm, mmu_seq))
> goto out_unlock;
> @@ -1781,7 +1790,7 @@ static int user_mem_abort(struct kvm_vcpu *vcpu, phys_addr_t fault_ipa,
> if (writable)
> kvm_set_pfn_dirty(pfn);
>
> - if (fault_status != FSC_PERM)
> + if (fault_status != FSC_PERM && !is_iomap(flags))
> clean_dcache_guest_page(pfn, vma_pagesize);
>
> if (exec_fault)
> @@ -1948,9 +1957,8 @@ int kvm_handle_guest_abort(struct kvm_vcpu *vcpu, struct kvm_run *run)
> if (kvm_is_error_hva(hva) || (write_fault && !writable)) {
> if (is_iabt) {
> /* Prefetch Abort on I/O address */
> - kvm_inject_pabt(vcpu, kvm_vcpu_get_hfar(vcpu));
> - ret = 1;
> - goto out_unlock;
> + ret = -ENOEXEC;
> + goto out;
> }
>
> /*
> @@ -1992,6 +2000,11 @@ int kvm_handle_guest_abort(struct kvm_vcpu *vcpu, struct kvm_run *run)
> ret = user_mem_abort(vcpu, fault_ipa, memslot, hva, fault_status);
> if (ret == 0)
> ret = 1;
> +out:
> + if (ret == -ENOEXEC) {
> + kvm_inject_pabt(vcpu, kvm_vcpu_get_hfar(vcpu));
> + ret = 1;
> + }
> out_unlock:
> srcu_read_unlock(&vcpu->kvm->srcu, idx);
> return ret;
> --
> 2.20.1
>
I can't seem to decide for myself if I think there's a sematic
difference between trying to execute from somewhere the VMM has
explicitly told us is device memory and from somewhere which we happen
to have mapped with VM_PFNMAP from user space. But I also can't seem to
really fault it (pun intended). Thoughts?
Thanks,
Christoffer
