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

[Will Deacon <will@kernel.org>]

On Mon, Feb 16, 2026 at 03:13:54PM +0000, James Clark wrote:
> 
> 
> On 16/02/2026 1:09 pm, Will Deacon 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.
> > 
> > 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();
> > +	}
> >   }
> >   static bool __trace_needs_switch(void)
> > @@ -79,15 +94,18 @@ static void __trace_switch_to_guest(void)
> >   	__trace_do_switch(host_data_ptr(host_debug_state.trfcr_el1),
> >   			  *host_data_ptr(trfcr_while_in_guest));
> > -
> > -	if (__trace_needs_drain()) {
> > -		isb();
> > -		tsb_csync();
> > -	}
> > +	__trace_drain_and_disable();
> >   }
> >   static void __trace_switch_to_host(void)
> >   {
> > +	u64 trblimitr_el1 = *host_data_ptr(host_debug_state.trblimitr_el1);
> > +
> > +	if (trblimitr_el1 & TRBLIMITR_EL1_E) {
> > +		write_sysreg_s(trblimitr_el1, SYS_TRBLIMITR_EL1);
> 
> Will this restore a stale value if you do kvm_enable_trbe() then later
> kvm_disable_trbe()? Looks like the read and save will be skipped unless
> host_data_test_flag(TRBE_ENABLED) is true, so it will never save a disabled
> value.

__trace_drain_and_disable() sets the saved limit to 0 if TRBE_ENABLE is
not set, so this shouldn't do anything in that case. Or did I
misunderstand your scenario?

> kvm_disable_trbe() might need to clear host_debug_state.trblimitr_el1.

pKVM can't rely on that thing being called, so the context switch still
needs to be self-contained there.

Will