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    SubjectRE: [PATCH] perf/x86/intel/uncore: Fix oops when counting IMC uncore events on some TGL
    Date
    From: Liang, Kan
    > Sent: 27 May 2020 15:47
    > On 5/27/2020 8:59 AM, David Laight wrote:
    > > From: kan.liang@linux.intel.com
    > >> Sent: 27 May 2020 13:31
    > >>
    > >> From: Kan Liang <kan.liang@linux.intel.com>
    > >>
    > >> When counting IMC uncore events on some TGL machines, an oops will be
    > >> triggered.
    > >> [ 393.101262] BUG: unable to handle page fault for address:
    > >> ffffb45200e15858
    > >> [ 393.101269] #PF: supervisor read access in kernel mode
    > >> [ 393.101271] #PF: error_code(0x0000) - not-present page
    > >>
    > >> Current perf uncore driver still use the IMC MAP SIZE inherited from
    > >> SNB, which is 0x6000.
    > >> However, the offset of IMC uncore counters for some TGL machines is
    > >> larger than 0x6000, e.g. 0xd8a0.
    > >>
    > >> Enlarge the IMC MAP SIZE for TGL to 0xe000.
    > >
    > > Replacing one 'random' constant with a different one
    > > doesn't seem like a proper fix.
    > >
    > > Surely the actual bounds of the 'memory' area are properly
    > > defined somewhere.
    > > Or at least should come from a table.
    > >
    > > You also need to verify that the offsets are within the mapped area.
    > > An unexpected offset shouldn't try to access an invalid address.
    >
    > Thanks for the review.
    >
    > I agree that we should add a check before mapping the area to prevent
    > the issue happens again.
    >
    > I think the check should be a generic check for all platforms which try
    > to map an area, not just for TGL. I will submit a separate patch for the
    > check.

    You need a check that the actual access is withing the mapped area.
    So instead of getting an OOPS you get a error.

    This is after you've mapped it.

    David

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