linux-kernel - [PATCH v6 2/4] mm: introduce new flag to indicate wc safe

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Date: Thu, 8 Feb 2024 02:16:50 +0530
From: <ankita@...dia.com>
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Subject: [PATCH v6 2/4] mm: introduce new flag to indicate wc safe

From: Ankit Agrawal <ankita@...dia.com>

Generalizing S2 setting from DEVICE_nGnRE to NormalNc for non PCI
devices may be problematic. E.g. GICv2 vCPU interface, which is
effectively a shared peripheral, can allow a guest to affect another
guest's interrupt distribution. The issue may be solved by limiting
the relaxation to mappings that have a user VMA. Still there is
insufficient information and uncertainity in the behavior of
non PCI drivers.

Add a new flag VM_VFIO_ALLOW_WC to indicate KVM that the device is
WC capable and these S2 changes can be extended to it. KVM can use
this flag to activate the code.

Signed-off-by: Ankit Agrawal <ankita@...dia.com>
Suggested-by: Catalin Marinas <catalin.marinas@....com>
Acked-by: Jason Gunthorpe <jgg@...dia.com>
---
 include/linux/mm.h | 14 ++++++++++++++
 1 file changed, 14 insertions(+)

diff --git a/include/linux/mm.h b/include/linux/mm.h
index f5a97dec5169..884c068a79eb 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -391,6 +391,20 @@ extern unsigned int kobjsize(const void *objp);
 # define VM_UFFD_MINOR		VM_NONE
 #endif /* CONFIG_HAVE_ARCH_USERFAULTFD_MINOR */
 
+/*
+ * This flag is used to connect VFIO to arch specific KVM code. It
+ * indicates that the memory under this VMA is safe for use with any
+ * non-cachable memory type inside KVM. Some VFIO devices, on some
+ * platforms, are thought to be unsafe and can cause machine crashes if
+ * KVM does not lock down the memory type.
+ */
+#ifdef CONFIG_64BIT
+#define VM_VFIO_ALLOW_WC_BIT	39
+#define VM_VFIO_ALLOW_WC	BIT(VM_VFIO_ALLOW_WC_BIT)
+#else
+#define VM_VFIO_ALLOW_WC	VM_NONE
+#endif
+
 /* Bits set in the VMA until the stack is in its final location */
 #define VM_STACK_INCOMPLETE_SETUP (VM_RAND_READ | VM_SEQ_READ | VM_STACK_EARLY)
 
-- 
2.34.1