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    Subject[PATCH v6 2/4] mm: introduce new flag to indicate wc safe
    Date
    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_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@nvidia.com>
    Suggested-by: Catalin Marinas <catalin.marinas@arm.com>
    Acked-by: Jason Gunthorpe <jgg@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..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

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