linux-kernel - [PATCH kernel v7 17/20] powerpc/powernv/npu: Fault user page into the hypervisor's pagetable

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Message-Id: <20181220082350.58113-18-aik@ozlabs.ru>
Date:   Thu, 20 Dec 2018 19:23:47 +1100
From:   Alexey Kardashevskiy <aik@...abs.ru>
To:     linuxppc-dev@...ts.ozlabs.org
Cc:     Alexey Kardashevskiy <aik@...abs.ru>,
        David Gibson <david@...son.dropbear.id.au>,
        kvm-ppc@...r.kernel.org, kvm@...r.kernel.org,
        Alistair Popple <alistair@...ple.id.au>,
        Reza Arbab <arbab@...ux.ibm.com>,
        Sam Bobroff <sbobroff@...ux.ibm.com>,
        Piotr Jaroszynski <pjaroszynski@...dia.com>,
        Leonardo Augusto Guimarães Garcia 
        <lagarcia@...ibm.com>, Jose Ricardo Ziviani <joserz@...ux.ibm.com>,
        Daniel Henrique Barboza <danielhb413@...il.com>,
        Alex Williamson <alex.williamson@...hat.com>,
        Paul Mackerras <paulus@...abs.org>,
        linux-kernel@...r.kernel.org, Christoph Hellwig <hch@...radead.org>
Subject: [PATCH kernel v7 17/20] 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@...abs.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