powerpc/powernv/npu: Fault user page into the hypervisor's pagetable

When a page fault happens in a GPU, the GPU signals the OS and the GPU
driver calls the fault handler which populated a page table; this allows
the GPU to complete an ATS request.

On the bare metal get_user_pages() is enough as it adds a pte to
the kernel page table but under KVM the partition scope tree does not get
updated so ATS will still fail.

This reads a byte from an effective address which causes HV storage
interrupt and KVM updates the partition scope tree.

Signed-off-by: Alexey Kardashevskiy <aik@ozlabs.ru>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

1 files changed, 7 insertions, 6 deletions

diff --git a/arch/powerpc/platforms/powernv/npu-dma.c b/arch/powerpc/platforms/powernv/npu-dma.c
index b713727663e2..d7f742ed48ba 100644
--- a/arch/powerpc/platforms/powernv/npu-dma.c
+++ b/arch/powerpc/platforms/powernv/npu-dma.c
@@ -1132,6 +1132,8 @@ int pnv_npu2_handle_fault(struct npu_context *context, uintptr_t *ea,
         u64 rc = 0, result = 0;
         int i, is_write;
         struct page *page[1];
+        const char __user *u;
+        char c;
 
         /* mmap_sem should be held so the struct_mm must be present */
         struct mm_struct *mm = context->mm;
@@ -1144,18 +1146,17 @@ int pnv_npu2_handle_fault(struct npu_context *context, uintptr_t *ea,
                                         is_write ? FOLL_WRITE : 0,
                                         page, NULL, NULL);
 
-                /*
-                 * To support virtualised environments we will have to do an
-                 * access to the page to ensure it gets faulted into the
-                 * hypervisor. For the moment virtualisation is not supported in
-                 * other areas so leave the access out.
-                 */
                 if (rc != 1) {
                         status[i] = rc;
                         result = -EFAULT;
                         continue;
                 }
 
+                /* Make sure partition scoped tree gets a pte */
+                u = page_address(page[0]);
+                if (__get_user(c, u))
+                        result = -EFAULT;
+
                 status[i] = 0;
                 put_page(page[0]);
         }