[PATCH v7 2/4] mm: introduce new flag to indicate wc safe

ankita
2024-02-11T17:47:46Z

From: Ankit Agrawal <ankita@nvidia.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_ALLOW_ANY_UNCACHED 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.

Suggested-by: Catalin Marinas <catalin.marinas@arm.com>
Reviewed-by: Jason Gunthorpe <jgg@nvidia.com>
Signed-off-by: Ankit Agrawal <ankita@nvidia.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..59576e56c58b 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_ALLOW_ANY_UNCACHED_BIT 39
+#define VM_ALLOW_ANY_UNCACHED BIT(VM_ALLOW_ANY_UNCACHED_BIT)
+#else
+#define VM_ALLOW_ANY_UNCACHED 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