[PATCH v3] arm/arm64: KVM: map MMIO regions at creation time

[Christoffer Dall]

On Thu, Oct 09, 2014 at 03:30:38PM +0200, Ard Biesheuvel wrote:
> There is really no point in faulting in memory regions page by page
> if they are not backed by demand paged system RAM but by a linear
> passthrough mapping of a host MMIO region. So instead, detect such
> regions at setup time and install the mappings for the backing all
> at once.
> 
> Signed-off-by: Ard Biesheuvel <ard.biesheuvel at linaro.org>
> ---
> 
> I have omitted the other 5 patches of the series of which this was #6, as
> Christoffer had indicated they could be merged separately.
> 
> Changes since v2:
> - moved the unmapping of moved/deleted regions to kvm_arch_flush_shadow_memslot
>   so it occurs before parts of the new regions may be mapped in
>   kvm_arch_prepare_memory_region
> - allow memory regions with holes
> 
> Changes since v1:
> - move this logic to kvm_arch_prepare_memory_region() so it can be invoked
>   when moving memory regions as well as when creating memory regions
> - as we are reasoning about memory regions now instead of memslots, all data
>   is retrieved from the 'mem' argument which points to a struct
>   kvm_userspace_memory_region
> - minor tweaks to the logic flow
> 
> My test case (UEFI under QEMU/KVM) still executes correctly with this patch,
> but more thorough testing with actual passthrough device regions is in order.
> 
>  arch/arm/kvm/mmu.c | 69 +++++++++++++++++++++++++++++++++++++++++++++++-------
>  1 file changed, 61 insertions(+), 8 deletions(-)
> 
> diff --git a/arch/arm/kvm/mmu.c b/arch/arm/kvm/mmu.c
> index 37c1b35f90ad..53d511524bb5 100644
> --- a/arch/arm/kvm/mmu.c
> +++ b/arch/arm/kvm/mmu.c
> @@ -1132,13 +1132,6 @@ void kvm_arch_commit_memory_region(struct kvm *kvm,
>  				   const struct kvm_memory_slot *old,
>  				   enum kvm_mr_change change)
>  {
> -	gpa_t gpa = old->base_gfn << PAGE_SHIFT;
> -	phys_addr_t size = old->npages << PAGE_SHIFT;
> -	if (change == KVM_MR_DELETE || change == KVM_MR_MOVE) {
> -		spin_lock(&kvm->mmu_lock);
> -		unmap_stage2_range(kvm, gpa, size);
> -		spin_unlock(&kvm->mmu_lock);
> -	}
>  }
>  
>  int kvm_arch_prepare_memory_region(struct kvm *kvm,
> @@ -1146,7 +1139,61 @@ int kvm_arch_prepare_memory_region(struct kvm *kvm,
>  				   struct kvm_userspace_memory_region *mem,
>  				   enum kvm_mr_change change)
>  {
> -	return 0;
> +	hva_t hva = mem->userspace_addr;
> +	hva_t reg_end = hva + mem->memory_size;
> +	int ret = 0;
> +
> +	if (change != KVM_MR_CREATE && change != KVM_MR_MOVE)
> +		return 0;
> +
> +	/*
> +	 * A memory region could potentially cover multiple VMAs, and any holes
> +	 * between them, so iterate over all of them to find out if we can map
> +	 * any of them right now.
> +	 *
> +	 *     +--------------------------------------------+
> +	 * +---------------+----------------+   +----------------+
> +	 * |   : VMA 1     |      VMA 2     |   |    VMA 3  :    |
> +	 * +---------------+----------------+   +----------------+
> +	 *     |               memory region                |
> +	 *     +--------------------------------------------+
> +	 */
> +	do {
> +		struct vm_area_struct *vma = find_vma(current->mm, hva);
> +		hva_t vm_start, vm_end;
> +
> +		if (!vma || vma->vm_start >= reg_end)
> +			break;
> +
> +		/*
> +		 * Take the intersection of this VMA with the memory region
> +		 */
> +		vm_start = max(hva, vma->vm_start);
> +		vm_end = min(reg_end, vma->vm_end);
> +
> +		if (vma->vm_flags & VM_PFNMAP) {
> +			gpa_t gpa = mem->guest_phys_addr +
> +				    (vm_start - mem->userspace_addr);
> +			phys_addr_t pa = (vma->vm_pgoff << PAGE_SHIFT) +
> +					 vm_start - vma->vm_start;
> +			bool writable = vma->vm_flags & VM_WRITE &&
> +					!(mem->flags & KVM_MEM_READONLY);

If I read the code correctly, in the case where you have (!(vma->vm_flags
& VM_WRITE) && !(mem->falgs & KVM_MEM_READONLY)) you'll map as read-only
and we'll take a Stage-2 fault on a write, but because the memslot is
not marked as readonly, we'll just try to fault in the page writable,
which should fail because (vma->vm_flags & VM_WRITE) == 0, so we'll
crash the VM here by returning -EFAULT to userspace.

So I'm wondering if this shouldn't return an error at this point
instead?

> +
> +			ret = kvm_phys_addr_ioremap(kvm, gpa, pa,
> +						    vm_end - vm_start,
> +						    writable);
> +			if (ret)
> +				break;
> +		}
> +		hva = vm_end;
> +	} while (hva < reg_end);
> +
> +	if (ret) {
> +		spin_lock(&kvm->mmu_lock);
> +		unmap_stage2_range(kvm, mem->guest_phys_addr, mem->memory_size);
> +		spin_unlock(&kvm->mmu_lock);
> +	}
> +	return ret;
>  }
>  
>  void kvm_arch_free_memslot(struct kvm *kvm, struct kvm_memory_slot *free,
> @@ -1171,4 +1218,10 @@ void kvm_arch_flush_shadow_all(struct kvm *kvm)
>  void kvm_arch_flush_shadow_memslot(struct kvm *kvm,
>  				   struct kvm_memory_slot *slot)
>  {
> +	gpa_t gpa = slot->base_gfn << PAGE_SHIFT;
> +	phys_addr_t size = slot->npages << PAGE_SHIFT;
> +
> +	spin_lock(&kvm->mmu_lock);
> +	unmap_stage2_range(kvm, gpa, size);
> +	spin_unlock(&kvm->mmu_lock);
>  }
> -- 
> 1.8.3.2
> 

Otherwise, this looks good.

Thanks!
-Christoffer