iommu/amd/pgtbl: Fix possible race while increase page table level

commit 1e56310b upstream.

The AMD IOMMU host page table implementation supports dynamic page table levels
(up to 6 levels), starting with a 3-level configuration that expands based on
IOVA address. The kernel maintains a root pointer and current page table level
to enable proper page table walks in alloc_pte()/fetch_pte() operations.

The IOMMU IOVA allocator initially starts with 32-bit address and onces its
exhuasted it switches to 64-bit address (max address is determined based
on IOMMU and device DMA capability). To support larger IOVA, AMD IOMMU
driver increases page table level.

But in unmap path (iommu_v1_unmap_pages()), fetch_pte() reads
pgtable->[root/mode] without lock. So its possible that in exteme corner case,
when increase_address_space() is updating pgtable->[root/mode], fetch_pte()
reads wrong page table level (pgtable->mode). It does compare the value with
level encoded in page table and returns NULL. This will result is
iommu_unmap ops to fail and upper layer may retry/log WARN_ON.

CPU 0                                         CPU 1
------                                       ------
map pages                                    unmap pages
alloc_pte() -> increase_address_space()      iommu_v1_unmap_pages() -> fetch_pte()
  pgtable->root = pte (new root value)
                                             READ pgtable->[mode/root]
					       Reads new root, old mode
  Updates mode (pgtable->mode += 1)

Since Page table level updates are infrequent and already synchronized with a
spinlock, implement seqcount to enable lock-free read operations on the read path.

Fixes: 754265bc ("iommu/amd: Fix race in increase_address_space()")
Reported-by: Alejandro Jimenez <alejandro.j.jimenez@oracle.com>
Cc: stable@vger.kernel.org
Cc: Joao Martins <joao.m.martins@oracle.com>
Cc: Suravee Suthikulpanit <suravee.suthikulpanit@amd.com>
Signed-off-by: Vasant Hegde <vasant.hegde@amd.com>
Signed-off-by: Joerg Roedel <joerg.roedel@amd.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>