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

James Clark
2026-02-16T15:13:58Z

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.

kvm_disable_trbe() might need to clear host_debug_state.trblimitr_el1.

> + isb();
> + }
> +
> __trace_do_switch(host_data_ptr(trfcr_while_in_guest),
> *host_data_ptr(host_debug_state.trfcr_el1));
> }