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

[Will Deacon <will@kernel.org>]

On Tue, Feb 17, 2026 at 02:19:17PM +0000, Leo Yan wrote:
> On Mon, Feb 16, 2026 at 06:14:11PM +0000, Will Deacon wrote:
> 
> [...]
> 
> > > 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.
> 
> Seems to me, this is correct.
> 
> > 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();
> 
> As I_QXJZX suggests, the section K10.5.10 "Context switching" gives
> the flow.  I'd suggest the VM context switch is also aligned to the
> description in S_VKHHY.

I honestly have a hard time believing the sequence in S_VKHHY as the DSB
seems to be in the wrong place which means the TSB CSYNC can float. It
also isn't aligned with what the EL1 driver does...

> When switching from host to guest, we need to clear TRCPRGCTLR.EN to
> zero.  As the doc states "ETE trace compression logic is stateful,
> and disabling the ETE resets this compression state".
> 
> > 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?
> 
> S_PKLXF gives the flow for switching in.

Well, modulo errata, sure. I don't have access to the errata document so
I was more interested in whether I got that right...

Will