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    SubjectRe: [Xen-devel] [PATCH] grant-table: don't set m2p override if kmap_ops is not set
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    On Tue, Nov 5, 2013 at 1:16 PM, Konrad Rzeszutek Wilk
    <konrad.wilk@oracle.com> wrote:
    > On Tue, Nov 05, 2013 at 12:53:17PM -0800, Anthony Liguori wrote:
    >> Matt Wilson <msw@linux.com> writes:
    >>
    >> > On Tue, Nov 05, 2013 at 05:03:58PM +0100, Roger Pau Monné wrote:
    >> >> On 05/11/13 16:08, Ian Campbell wrote:
    >> >> > On Tue, 2013-11-05 at 16:01 +0100, Roger Pau Monné wrote:
    >> >> >> On 05/11/13 15:56, Konrad Rzeszutek Wilk wrote:
    >> >> >>> On Tue, Nov 05, 2013 at 03:47:08PM +0100, Roger Pau Monné wrote:
    >> >> >>>> On 05/11/13 13:36, David Vrabel wrote:
    >> >> >>>>> On 05/11/13 11:24, Roger Pau Monne wrote:
    >> >> >>>>>> IMHO there's no reason to set a m2p override if the mapping is done in
    >> >> >>>>>> kernel space, so only set the m2p override when kmap_ops is set.
    >> >> >>>>>
    >> >> >>>>> Can you provide a more detailed reasoning about why this is safe?
    >> >> >>>>
    >> >> >>>> To tell the truth, I don't understand why we need to use the m2p
    >> >> >>>> override for kernel space only mappings, my understanding is that this
    >> >> >>>> m2p override is needed for user space mappings only (where we actually
    >> >> >>>> end up doing two mappings, one in kernel space and one in user space).
    >> >> >>>> For kernel space I don't see why we need to do anything else than
    >> >> >>>> setting the right p2m translation.
    >> >> >>>
    >> >> >>> We needed the m2p when doing DMA operations. As the driver would
    >> >> >>> want the bus address (so p2m) and then when unmapping the DMA we
    >> >> >>> only get the bus address - so we needed to do a m2p lookup.
    >> >> >>
    >> >> >> OK, we need a m2p (that we already have in machine_to_phys_mapping),
    >> >> >> what I don't understand is why we need the m2p override.
    >> >> >
    >> >> > The m2p is a host global table.
    >> >> >
    >> >> > For a foreign page grant mapped into the current domain the m2p will
    >> >> > give you the foreign (owner) domain's p from the m, not the local one.
    >> >>
    >> >> Yes, you are completely right, then I have to figure out why blkback
    >> >> works fine with this patch applied (or at least it seems to work fine).
    >> >
    >> > blkback also works for me when testing a similar patch. I'm still
    >> > confused. One thing with your proposed patch: I'm not sure that you're
    >> > putting back the correct mfn.
    >>
    >> It's perfectly fine to store a foreign pfn in the m2p table. The m2p
    >> override table is used by the grant device to allow a reverse lookup of
    >> the real mfn to a pfn even if it's foreign.
    >>
    >> blkback doesn't actually need this though. This was introduced in:
    >>
    >> commit 5dc03639cc903f887931831d69895facb5260f4b
    >> Author: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
    >> Date: Tue Mar 1 16:46:45 2011 -0500
    >>
    >> xen/blkback: Utilize the M2P override mechanism for GNTMAP_host_map
    >>
    >> Purely as an optimization. In practice though due to lock contention it
    >> slows things down.
    >>
    >> I think an alternative would be to use a read/write lock instead of just
    >> a spinlock since it's the read path that is the most hot.
    >
    > The m2p hash table can also be expanded to lower the contention.

    That was the first thing I tried :-)

    The issue isn't lookup cost as much as the number of acquisitions and
    the cost of adding/removing entries.

    >> I haven't tested that yet though.
    >
    > Looking forward to your patches :-)

    I'm really just being thorough in suggesting the rwlock. I actually
    think that not using the m2p override table is the right long term
    fix.

    Regards,

    Anthony Liguori

    >>
    >> Regards,
    >>
    >> Anthony Liguori
    >>
    >> >
    >> > Adding Anthony to the thread.
    >> >
    >> > --msw
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