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    SubjectRe: [PATCH v5 07/13] block: Add PCI P2P flag for request queue and check support for requests


    On 05/09/18 02:36 PM, Jens Axboe wrote:
    > On 9/5/18 2:32 PM, Logan Gunthorpe wrote:
    >>
    >>
    >> On 05/09/18 02:19 PM, Jens Axboe wrote:
    >>> On 9/5/18 2:18 PM, Logan Gunthorpe wrote:
    >>>>
    >>>>
    >>>> On 05/09/18 02:14 PM, Jens Axboe wrote:
    >>>>> But if the caller must absolutely know where the bio will end up, then
    >>>>> it seems super redundant. So I'd vote for killing this check, it buys
    >>>>> us absolutely nothing and isn't even exhaustive in its current form.
    >>>>
    >>>>
    >>>> Ok, I'll remove it for v6.
    >>>
    >>> Since the drivers needs to know it's doing it right, it might not
    >>> hurt to add a sanity check helper for that. Just have the driver
    >>> call it, and don't add it in the normal IO submission path.
    >>
    >> I'm not sure I really see the value in that. It's the same principle in
    >> asking the driver to do the WARN: if the developer knew enough to use
    >> the special helper, they probably knew well enough to do the rest correctly.
    >
    > I don't agree with that at all. It's a "is my request valid" helper,
    > it's not some obscure and rarely used functionality. You're making up
    > this API right now, if you really want it done for every IO, make it
    > part of the p2p submission process. You could even hide it behind a
    > debug thing, if you like.

    There is no special p2p submission process. In the nvme-of case we are
    using the existing process and with the code in blk-core it didn't
    change it's process at all. Creating a helper will create one and I can
    look at making a pci_p2pdma_submit_bio() for v6; but if the developer
    screws up and still calls the regular submit_bio() things will only be
    very subtly broken and that won't be obvious.

    Logan

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