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

Marc Zyngier
2026-02-16T15:51:05Z

On Mon, 16 Feb 2026 15:05:10 +0000,
James Clark <james.clark@linaro.org> wrote:
>
>
>
> On 16/02/2026 2:29 pm, Marc Zyngier wrote:
> > On Mon, 16 Feb 2026 13:09:59 +0000,
> > Will Deacon <will@kernel.org> wrote:
> >>
> >> The nVHE world-switch code relies on zeroing TRFCR_EL1 to disable trace
> >> generation in guest context when self-hosted TRBE is in use by the host.
> >>
> >> Per D3.2.1 ("Controls to prohibit trace at Exception levels"), clearing
> >> TRFCR_EL1 means that trace generation is prohibited at EL1 and EL0 but
> >> per R_YCHKJ the Trace Buffer Unit will still be enabled if
> >> TRBLIMITR_EL1.E is set. R_SJFRQ goes on to state that, when enabled, the
> >> Trace Buffer Unit can perform address translation for the "owning
> >> exception level" even when it is out of context.
> >
> > Great. So TRBE violates all the principles that we hold true in the
> > architecture. Does SPE suffer from the same level of brokenness?
> >
> >> Consequently, we can end up in a state where TRBE performs speculative
> >> page-table walks for a host VA/IPA in guest/hypervisor context depending
> >> on the value of MDCR_EL2.E2TB, which changes over world-switch. The
> >> result appears to be a heady mixture of data corruption and hardware
> >> lockups.
> >>
> >> Extend the TRBE world-switch code to clear TRBLIMITR_EL1.E after
> >> draining the buffer, restoring the register on return to the host.
> >>
> >> Cc: Marc Zyngier <maz@kernel.org>
> >> Cc: Oliver Upton <oupton@kernel.org>
> >> Cc: James Clark <james.clark@linaro.org>
> >> Cc: Leo Yan <leo.yan@arm.com>
> >> Cc: Suzuki K Poulose <suzuki.poulose@arm.com>
> >> Cc: Fuad Tabba <tabba@google.com>
> >> Fixes: a1319260bf62 ("arm64: KVM: Enable access to TRBE support for host")
> >> Signed-off-by: Will Deacon <will@kernel.org>
> >> ---
> >>
> >> NOTE: This is *untested* as I don't have a TRBE-capable device that can
> >> run upstream but I noticed this by inspection when triaging occasional
> >> hardware lockups on systems using a 6.12-based kernel with TRBE running
> >> at the same time as a vCPU is loaded. This code has changed quite a bit
> >> over time, so stable backports are not entirely straightforward.
> >> Hopefully James/Leo/Suzuki can help us test if folks agree with the
> >> general approach taken here.
> >>
> >> arch/arm64/include/asm/kvm_host.h | 1 +
> >> arch/arm64/kvm/hyp/nvhe/debug-sr.c | 36 ++++++++++++++++++++++--------
> >> 2 files changed, 28 insertions(+), 9 deletions(-)
> >>
> >> diff --git a/arch/arm64/include/asm/kvm_host.h b/arch/arm64/include/asm/kvm_host.h
> >> index ac7f970c7883..a932cf043b83 100644
> >> --- a/arch/arm64/include/asm/kvm_host.h
> >> +++ b/arch/arm64/include/asm/kvm_host.h
> >> @@ -746,6 +746,7 @@ struct kvm_host_data {
> >> u64 pmscr_el1;
> >> /* Self-hosted trace */
> >> u64 trfcr_el1;
> >> + u64 trblimitr_el1;
> >> /* Values of trap registers for the host before guest entry. */
> >> u64 mdcr_el2;
> >> u64 brbcr_el1;
> >> diff --git a/arch/arm64/kvm/hyp/nvhe/debug-sr.c b/arch/arm64/kvm/hyp/nvhe/debug-sr.c
> >> index 2a1c0f49792b..fd389a26bc59 100644
> >> --- a/arch/arm64/kvm/hyp/nvhe/debug-sr.c
> >> +++ b/arch/arm64/kvm/hyp/nvhe/debug-sr.c
> >> @@ -57,12 +57,27 @@ static void __trace_do_switch(u64 *saved_trfcr, u64 new_trfcr)
> >> write_sysreg_el1(new_trfcr, SYS_TRFCR);
> >> }
> >> -static bool __trace_needs_drain(void)
> >> +static void __trace_drain_and_disable(void)
> >> {
> >> - if (is_protected_kvm_enabled() && host_data_test_flag(HAS_TRBE))
> >> - return read_sysreg_s(SYS_TRBLIMITR_EL1) & TRBLIMITR_EL1_E;
> >> + u64 *trblimitr_el1 = host_data_ptr(host_debug_state.trblimitr_el1);
> >> - return host_data_test_flag(TRBE_ENABLED);
> >> + *trblimitr_el1 = 0;
> >> +
> >> + if (is_protected_kvm_enabled()) {
> >> + if (!host_data_test_flag(HAS_TRBE))
> >> + return;
> >> + } else {
> >> + if (!host_data_test_flag(TRBE_ENABLED))
> >> + return;
> >> + }
> >> +
> >> + *trblimitr_el1 = read_sysreg_s(SYS_TRBLIMITR_EL1);
> >> + if (*trblimitr_el1 & TRBLIMITR_EL1_E) {
> >> + isb();
> >> + tsb_csync();
> >> + write_sysreg_s(0, SYS_TRBLIMITR_EL1);
> >> + isb();
>
> The TRBE driver might do an extra drain here as a workaround. Hard to
> tell if it's actually required in this case (seems like probably not)
> but it might be worth doing it anyway to avoid hitting the
> issue. Especially if we add guest support later where some of the
> affected registers might start being used.

Just to set the expectations: guest TRBE support is not happening
until the architecture is fixed. It cannot reliably give a trace that
includes emulated exceptions, and until then, no TRBE for you.

> See:
>
> if (trbe_needs_drain_after_disable(cpudata))
> trbe_drain_buffer();
>
>
> >> + }
> >
> > Doesn't this mean we should be able to get rid of most of the TRFCR
> > messing about that litters the entry/exit code and leave that to VHE
>
> Technically you could have ETMs that and are connected to sinks other
> than TRBE. Unless you somehow switch off those sinks you still need to
> do the TRFCR switching stuff.
>
> > only? And even then, I'm tempted to simply get rid of any sort of
> > guest-only tracing, given that TRBE is not capable of representing
> > exceptions that are synthesised by the host, making it the resulting
> > traces useless.
>
> I haven't heard of anyone tracing a guest from the host, but until we
> add support for guests to be able to trace themselves it's the only
> way of doing it, so it could be useful.

But that's *not* working. If you trace EL1 only, even with a VHE host,
the result is not usable.

> Although all the messing
> around with TRFCR before was from a request to disable guest trace
> rather than enable it, as it was always on in nVHE.

I can't see how it could ever be "always on", unless you were happy to
randomly corrupt memory?

> > I'm still trying to get my hands on a TRBE-enabled system that has
> > some actual firmware tables (my O6 seems to have the HW, but no
> > description of the required coresight infra).
>
> Leo has a guide, I haven't tried it myself yet but it should be
> working. I'll send it to you.

Thanks. But why isn't that stuff publicly documented? Sight...

M.

--
Without deviation from the norm, progress is not possible.