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    SubjectRe: [PATCH] x86: define arch_vm_get_page_prot to set _PAGE_IOMAP on VM_IO vmas
    On 10/22/2010 08:08 AM, Konrad Rzeszutek Wilk wrote:
    >> Okay, could you clarify this part a bit? Why does the kernel need to
    >> know the difference between "pseudo-physical" and "machine addresses" at
    >> all? If they overlap, there is a problem, and if they don't overlap, it
    >> will be a 1:1 mapping anyway...
    > The flag (_PAGE_IOMAP) is used when we set the PTE so that the MFN value is
    > used instead of the PFN. We need that b/c when a driver does page_to_pfn()
    > it ends up using the PFN as bus address to write out registers data.
    > Without this patch, the page->virt->PFN value is used and the PFN != to real MFN
    > so we end up writing in a memory address that the PCI device has no idea about.
    > By setting the PTE with the MFN, the virt->PFN gets the real MFN value.
    > The drivers I am talking about are mostly, if not all, located in drivers/gpu
    > and it looks that we are missing two more patches to utilize the patch
    > that Jeremy posted.
    > Please note that I am _not_ suggesting that the two patches
    > below should go out - I still need to post them on drm mailing list.

    I'm still seriously confused. If I understand this correctly, we're
    talking about DMA addresses here (as opposed to PIO addresses, i.e.
    BARs), right?

    It's the bimodality that really bothers me. I understand of course that
    Xen imposes yet another address remapping layer, but I'm having a hard
    time understanding any conditions under with we would need that layer to
    go away, as long as DMA addresses are translated via the DMA APIs -- and
    if they aren't, then iommus will break, too.

    As such, I don't grok this page flag and what it does, and why it's
    needed. I'm not saying it's *wrong*, I'm saying the design is opaque to
    me and I'm not sure it is the right solution.


    H. Peter Anvin, Intel Open Source Technology Center
    I work for Intel. I don't speak on their behalf.

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