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Subject[PATCH kernel v7 17/20] powerpc/powernv/npu: Fault user page into the hypervisor's pagetable
Date
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>
---
arch/powerpc/platforms/powernv/npu-dma.c | 13 +++++++------
1 file changed, 7 insertions(+), 6 deletions(-)

diff --git a/arch/powerpc/platforms/powernv/npu-dma.c b/arch/powerpc/platforms/powernv/npu-dma.c
index c6163b9..12b8421 100644
--- a/arch/powerpc/platforms/powernv/npu-dma.c
+++ b/arch/powerpc/platforms/powernv/npu-dma.c
@@ -1133,6 +1133,8 @@ 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;
@@ -1145,18 +1147,17 @@ 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]);
}
--
2.17.1
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