[PATCH v2 0/1] KVM: arm64: fix the mmio faulting

[Santosh Shukla <sashukla@nvidia.com>]

Description of the Reproducer scenario as asked in the thread [1].

Tried to create the reproducer scenario with vfio-pci driver using
nvidia GPU in PT mode, As because vfio-pci driver now supports
vma faulting (/vfio_pci_mmap_fault) so could create a crude reproducer
situation with that.

To create the repro - I did an ugly hack into arm64/kvm/mmu.c.
The hack is to make sure that stage2 mapping are not created
at the time of vm_init by unsetting VM_PFNMAP flag. This `unsetting` flag
needed because vfio-pci's mmap func(/vfio_pci_mmap) by-default
sets the VM_PFNMAP flag for the MMIO region but I want
the remap_pfn_range() func to set the _PFNMAP flag via vfio's fault
handler func vfio_pci_mmap_fault().

So with above, when guest access the MMIO region, this will
trigger the mmio fault path at arm64-kvm hypervisor layer like below:
user_mem_abort() {->...
    --> Check the VM_PFNMAP flag, since not set so marks force_pte=false
    ....
    __gfn_to_pfn_memslot()-->
    ...
    handle_mm_fault()-->
    do_fault()-->
    vfio_pci_mmio_fault()-->
    remap_pfn_range()--> Now will set the VM_PFNMAP flag.
}

Since the force_pte flag is set to false so will lead to THP oops.
By setting the force_pte=true will avoid the THP Oops which was
mentioned in the [2] and patch proposition [1/1] fixes that.

hackish change to reproduce scenario:
--->
diff --git a/arch/arm64/kvm/mmu.c b/arch/arm64/kvm/mmu.c
index d4cd25334610..b0a999aa6a95 100644
--- a/arch/arm64/kvm/mmu.c
+++ b/arch/arm64/kvm/mmu.c
@@ -1318,6 +1318,12 @@ int kvm_arch_prepare_memory_region(struct kvm *kvm,
 		vm_start = max(hva, vma->vm_start);
 		vm_end = min(reg_end, vma->vm_end);
 
+		/* Hack to make sure stage2 mapping not present, thus trigger
+		 * user_mem_abort for stage2 mapping
+		 */
+		if (vma->vm_flags & VM_PFNMAP) {
+			vma->vm_flags = vma->vm_flags & (~VM_PFNMAP);
+		}
 		if (vma->vm_flags & VM_PFNMAP) {
 			gpa_t gpa = mem->guest_phys_addr +
 				    (vm_start - mem->userspace_addr);


Thanks.
Santosh

[1] https://lkml.org/lkml/2020/10/23/310
[2] https://lkml.org/lkml/2020/10/21/460


Santosh Shukla (1):
  KVM: arm64: Correctly handle the mmio faulting

 arch/arm64/kvm/mmu.c | 1 +
 1 file changed, 1 insertion(+)

-- 
2.7.4

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