lkml.org 
[lkml]   [2013]   [Nov]   [6]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
    /
    Date
    From
    SubjectRe: [Xen-devel] [PATCH] grant-table: don't set m2p override if kmap_ops is not set
    On 05/11/13 20:53, 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.

    No. A m2p lookup should not be needed for DMA unmap.

    We only need the p2m lookup on DMA map to get the bus address -- there's
    should be nothing to do on the unmap.

    However, there is dma_mark_clean(phys_to_virt(phys), size) call in
    xen_unmap_single() which would require the m2p lookup but this call is
    not necessary as dma_mark_clean() is a no-op for anything except ia64
    and we do not care about ia64.

    >>>>> 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.

    Matt, Anthony, I presume you have profiling results or performance data
    that support this proposed change? Can you provide them?

    > 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.

    The full changeset description for this change doesn't make sense to me.

    xen/blkback: Utilize the M2P override mechanism for GNTMAP_host_map

    Instead of doing copy grants lets do mapping grants using
    the M2P(and P2M) override mechanism.

    As all it is doing is replacing set_phys_to_machine() calls with
    m2p_add_override().

    David
    --
    To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
    the body of a message to majordomo@vger.kernel.org
    More majordomo info at http://vger.kernel.org/majordomo-info.html
    Please read the FAQ at http://www.tux.org/lkml/

    \
     
     \ /
      Last update: 2013-11-06 13:01    [W:2.534 / U:0.132 seconds]
    ©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site