• Julien Grall's avatar
    xen: Make clear that swiotlb and biomerge are dealing with DMA address · 32e09870
    Julien Grall authored
    
    
    The swiotlb is required when programming a DMA address on ARM when a
    device is not protected by an IOMMU.
    
    In this case, the DMA address should always be equal to the machine address.
    For DOM0 memory, Xen ensure it by have an identity mapping between the
    guest address and host address. However, when mapping a foreign grant
    reference, the 1:1 model doesn't work.
    
    For ARM guest, most of the callers of pfn_to_mfn expects to get a GFN
    (Guest Frame Number), i.e a PFN (Page Frame Number) from the Linux point
    of view given that all ARM guest are auto-translated.
    
    Even though the name pfn_to_mfn is misleading, we need to ensure that
    those caller get a GFN and not by mistake a MFN. In pratical, I haven't
    seen error related to this but we should fix it for the sake of
    correctness.
    
    In order to fix the implementation of pfn_to_mfn on ARM in a follow-up
    patch, we have to introduce new helpers to return the DMA from a PFN and
    the invert.
    
    On x86, the new helpers will be an alias of pfn_to_mfn and mfn_to_pfn.
    
    The helpers will be used in swiotlb and xen_biovec_phys_mergeable.
    
    This is necessary in the latter because we have to ensure that the
    biovec code will not try to merge a biovec using foreign page and
    another using Linux memory.
    
    Lastly, the helper mfn_to_local_pfn has been renamed to bfn_to_local_pfn
    given that the only usage was in swiotlb.
    Signed-off-by: default avatarJulien Grall <julien.grall@citrix.com>
    Reviewed-by: default avatarStefano Stabellini <stefano.stabellini@eu.citrix.com>
    Signed-off-by: default avatarDavid Vrabel <david.vrabel@citrix.com>
    32e09870