RE: [PATCH] perf/x86/intel/uncore: Fix oops when counting IMC uncore events on some TGL

[David Laight]

From: Liang, Kan
> Sent: 27 May 2020 16:01
> On 5/27/2020 10:51 AM, David Laight wrote:
> > From: Liang, Kan
> >> Sent: 27 May 2020 15:47
> >> On 5/27/2020 8:59 AM, David Laight wrote:
> >>> From: kan.liang@xxxxxxxxxxxxxxx
> >>>> Sent: 27 May 2020 13:31
> >>>>
> >>>> From: Kan Liang <kan.liang@xxxxxxxxxxxxxxx>
> >>>>
> >>>> 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.
> 
> Sure. Will add a WARN_ONCE() before the actual access.

No that will still panic some systems.
pr_warn() is all you need.

	David

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