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 03:05:10PM +0000, James Clark 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:
> > > 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. See:
> 
>     if (trbe_needs_drain_after_disable(cpudata))
>         trbe_drain_buffer();

Oh great, this thing sucks even more than I realised!

But thanks for pointing that out... this is presumably erratum #2064142,
but we probably need to look at #2038923 as well :/

I can't find any public documentation for the problems, but based on the
kconfig text then I think we care about #2064142 so that the TRBE
register writes when restoring the host context are effective and we
care about #2038923 to avoid corrupting trace when re-enabling for the
host.

It also looks like we can't rely on the dsb(nsh) in the vcpu_run()
path if that needs to be before the write to TRBLIMITR_EL1.

In which case, the host->guest something hideous like:

	isb();
	tsb_csync();	// Executes twice if ARM64_WORKAROUND_TSB_FLUSH_FAILURE!
	dsb(nsh);	// I missed this in my patch
	write_sysreg_s(0, SYS_TRBLIMITR_EL1);
	if (2064142) {
		tsb_csync();
		dsb(nsh);
	}
	isb();

and then the guest->host part is:

	write_sysreg_s(trblimitr_el1, SYS_TRBLIMITR_EL1);
	isb();
	if (2038923)
		isb();

Does that look right to you?

Will