Re: [PATCH] coresight: trbe: Move check for kernelspace unmapped at EL0 to probe

[Suzuki K Poulose]

On 03/02/2022 12:04, Sudeep Holla wrote:
> On Thu, Feb 03, 2022 at 11:55:58AM +0530, Anshuman Khandual wrote:
> > On 2/1/22 5:52 PM, Sudeep Holla wrote:
> > > Currently with the check present in the module initialisation, it shouts
> > > on all the systems irrespective of presence of coresight trace buffer
> > > extensions.
> >
> > IIUC a system with CONFIG_CORESIGHT_TRBE enabled but without a TRBE DT
> > i.e "arm,trace-buffer-extension" complains about kernel space unmapping
> > at EL0 (even though it does not even really have TRBE HW to initialize).
>
>
> Correct. Basically, this error will be seen on all systems(DT and ACPI) when
> the module is compiled. It really doesn't matter if the system supports TRBE.
>
> > > Similar to Arm SPE perf driver, move the check for kernelspace unmapping
> > > when running at EL0 to the device probe instead of module initialisation.
> >
> > Makes sense.
>
> Thanks.
>
> > > Cc: Mathieu Poirier <mathieu.poirier@xxxxxxxxxx>
> > > Cc: Suzuki K Poulose <suzuki.poulose@xxxxxxx>
> > > Cc: Mike Leach <mike.leach@xxxxxxxxxx>
> > > Cc: Leo Yan <leo.yan@xxxxxxxxxx>
> > > Cc: Anshuman Khandual <anshuman.khandual@xxxxxxx>
> > > Cc: coresight@xxxxxxxxxxxxxxxx
> > > Signed-off-by: Sudeep Holla <sudeep.holla@xxxxxxx>
> > > ---
> > >   drivers/hwtracing/coresight/coresight-trbe.c | 10 +++++-----
> > >   1 file changed, 5 insertions(+), 5 deletions(-)
> > >
> > > diff --git a/drivers/hwtracing/coresight/coresight-trbe.c b/drivers/hwtracing/coresight/coresight-trbe.c
> > > index 276862c07e32..3fe2ce1ba5bf 100644
> > > --- a/drivers/hwtracing/coresight/coresight-trbe.c
> > > +++ b/drivers/hwtracing/coresight/coresight-trbe.c
> > > @@ -1423,6 +1423,11 @@ static int arm_trbe_device_probe(struct platform_device *pdev)
> > >   	struct device *dev = &pdev->dev;
> > >   	int ret;
> > >   
> >
> > Could you please add a similar comment like SPE driver regarding how
> > the TRBE buffer will be inaccessible, if kernel gets unmapped at EL0
> > and trace capture will terminate.
>
> Sure I can add that. But if the device probe fails, will you be able to even
> start the trace capture, sorry I didn't get what you mean by "trace capture
> will terminate". I assume it must be "trace capture is not possible or not
> allowed" IIUC.

"Trace capture is not possible with kernel page table isolation"

 is good enough.

Thanks for making these changes

Cheers
Suzuki