Re: [PATCH] KVM: arm64: Disable TRBE Trace Buffer Unit when running in guest context

[Leo Yan <leo.yan@arm.com>]

On Fri, Feb 27, 2026 at 06:07:44PM +0000, Will Deacon wrote:
> On Wed, Feb 25, 2026 at 12:09:56PM +0000, Leo Yan wrote:
> > [ + Yabin ]
> > 
> > Thanks for Suzuki's reminding, I should mention that Yabin reported
> > another lockup issue caused by missing CPU PM support in TRBE driver.
> > 
> > We have a patch series to fix the issue:
> > https://lore.kernel.org/linux-arm-kernel/20251119-arm_coresight_path_power_management_improvement-v5-16-f615a301ad0b@arm.com/
> 
> Two nits on that series:
> 
> 1. It seems a bit weird to me for the ETE driver to manage TRFCR but for
>    the TRBE driver to manage the other registers

TRFCR_ELx is introduced by FEAT_TRF, which is a separate feature from
TRBE, and it can be used for other sinks (like ETR).  I think this is
the main reason that it is implemented in ETE driver rather than TRBE
driver.

> 2. Are you sure you don't need to save/restore the TRBE state when
>    LIMITR.E is clear? Maybe the driver is fine with that, but I'm worried
>    that we could suspend in a half-programmed state and lose some of that
>    configuration.

If the TRBLIMITR_EL1.E bit is cleared during the CPU is idle, then the
next time the TRBE trace buffer is re-enabled, trbe_enable_hw() must be
called to reconfigure the TRBE registers (including TRBSR_EL1).

One concern is that after a CPU power cycle, some fields in the TRBE
registers may be in the following state:

  "On a cold reset, this field resets to an architecturally UNKNOWN value."

I will change to always save/restore TRBE state.  Thanks for
suggestions.

> > Besides your fix the translation regime issue, I'd also suggest applying
> > the CoreSight PM patch series to fix lockup caused by CPU idle.
> 
> Yes, we definitely need something like that in the android kernel trees.
> I've previously bodged a hack into the ETE PM notifiers, but if you have
> backports of your series to 6.12, 6.6 and 6.1 then we should merge them
> into Android. As it stands, I don't have a TRBE-capable device running
> mainline.

Let us first merge the series on the master :)

After that, we can consider backporting (I assume Yabin already has a
plan for this).  I'd be happy to help with backporting to v6.12.
However, I cannot commit to backporting to v6.6 or v6.1 at this stage,
as many dependencies are likely to be involved.

Thanks,
Leo