Re: [PATCH v5 07/19] arm64: kvm: Enable access to TRBE support for host

[Marc Zyngier <maz@kernel.org>]

On Tue, 30 Mar 2021 12:12:49 +0100,
Suzuki K Poulose <suzuki.poulose@arm.com> wrote:
> 
> Hi Marc
> 
> On 30/03/2021 11:12, Marc Zyngier wrote:
> > Hi Suzuki,
> > 
> > [+ Alex]
> > 
> > On Tue, 23 Mar 2021 12:06:35 +0000,
> > Suzuki K Poulose <suzuki.poulose@arm.com> wrote:
> >> 
> >> For a nvhe host, the EL2 must allow the EL1&0 translation
> >> regime for TraceBuffer (MDCR_EL2.E2TB == 0b11). This must
> >> be saved/restored over a trip to the guest. Also, before
> >> entering the guest, we must flush any trace data if the
> >> TRBE was enabled. And we must prohibit the generation
> >> of trace while we are in EL1 by clearing the TRFCR_EL1.
> >> 
> >> For vhe, the EL2 must prevent the EL1 access to the Trace
> >> Buffer.
> >> 
> >> Cc: Will Deacon <will@kernel.org>
> >> Cc: Catalin Marinas <catalin.marinas@arm.com>
> >> Cc: Marc Zyngier <maz@kernel.org>
> >> Cc: Mark Rutland <mark.rutland@arm.com>
> >> Cc: Anshuman Khandual <anshuman.khandual@arm.com>
> >> Acked-by: Mathieu Poirier <mathieu.poirier@linaro.org>
> >> Signed-off-by: Suzuki K Poulose <suzuki.poulose@arm.com>
> >> ---
> >>   arch/arm64/include/asm/el2_setup.h | 13 +++++++++
> >>   arch/arm64/include/asm/kvm_arm.h   |  2 ++
> >>   arch/arm64/include/asm/kvm_host.h  |  2 ++
> >>   arch/arm64/kernel/hyp-stub.S       |  3 ++-
> >>   arch/arm64/kvm/debug.c             |  6 ++---
> >>   arch/arm64/kvm/hyp/nvhe/debug-sr.c | 42 ++++++++++++++++++++++++++++++
> >>   arch/arm64/kvm/hyp/nvhe/switch.c   |  1 +
> >>   7 files changed, 65 insertions(+), 4 deletions(-)
> >> 
> >> diff --git a/arch/arm64/include/asm/el2_setup.h b/arch/arm64/include/asm/el2_setup.h
> >> index d77d358f9395..bda918948471 100644
> >> --- a/arch/arm64/include/asm/el2_setup.h
> >> +++ b/arch/arm64/include/asm/el2_setup.h
> >> @@ -65,6 +65,19 @@
> >>   						// use EL1&0 translation.
> >>     .Lskip_spe_\@:
> >> +	/* Trace buffer */
> >> +	ubfx	x0, x1, #ID_AA64DFR0_TRBE_SHIFT, #4
> >> +	cbz	x0, .Lskip_trace_\@		// Skip if TraceBuffer is not present
> >> +
> >> +	mrs_s	x0, SYS_TRBIDR_EL1
> >> +	and	x0, x0, TRBIDR_PROG
> >> +	cbnz	x0, .Lskip_trace_\@		// If TRBE is available at EL2
> >> +
> >> +	mov	x0, #(MDCR_EL2_E2TB_MASK << MDCR_EL2_E2TB_SHIFT)
> >> +	orr	x2, x2, x0			// allow the EL1&0 translation
> >> +						// to own it.
> >> +
> >> +.Lskip_trace_\@:
> >>   	msr	mdcr_el2, x2			// Configure debug traps
> >>   .endm
> >>   diff --git a/arch/arm64/include/asm/kvm_arm.h
> >> b/arch/arm64/include/asm/kvm_arm.h
> >> index 94d4025acc0b..692c9049befa 100644
> >> --- a/arch/arm64/include/asm/kvm_arm.h
> >> +++ b/arch/arm64/include/asm/kvm_arm.h
> >> @@ -278,6 +278,8 @@
> >>   #define CPTR_EL2_DEFAULT	CPTR_EL2_RES1
> >>     /* Hyp Debug Configuration Register bits */
> >> +#define MDCR_EL2_E2TB_MASK	(UL(0x3))
> >> +#define MDCR_EL2_E2TB_SHIFT	(UL(24))
> > 
> > Where are these bits defined? DDI0487G_a has them as RES0.
> 
> They are part of the Future architecture technology and a register
> definition XML is available here :
> 
> https://developer.arm.com/documentation/ddi0601/2020-12/AArch64-Registers/MDCR-EL2--Monitor-Debug-Configuration-Register--EL2-?lang=en#fieldset_0-25_24-1
>

It be worth adding a pointer to that documentation until this is part
of a released ARM ARM.

> > 
> >>   #define MDCR_EL2_TTRF		(1 << 19)
> >>   #define MDCR_EL2_TPMS		(1 << 14)
> >>   #define MDCR_EL2_E2PB_MASK	(UL(0x3))
> >> diff --git a/arch/arm64/include/asm/kvm_host.h b/arch/arm64/include/asm/kvm_host.h
> >> index 3d10e6527f7d..80d0a1a82a4c 100644
> >> --- a/arch/arm64/include/asm/kvm_host.h
> >> +++ b/arch/arm64/include/asm/kvm_host.h
> >> @@ -315,6 +315,8 @@ struct kvm_vcpu_arch {
> >>   		struct kvm_guest_debug_arch regs;
> >>   		/* Statistical profiling extension */
> >>   		u64 pmscr_el1;
> >> +		/* Self-hosted trace */
> >> +		u64 trfcr_el1;
> >>   	} host_debug_state;
> >>     	/* VGIC state */
> >> diff --git a/arch/arm64/kernel/hyp-stub.S b/arch/arm64/kernel/hyp-stub.S
> >> index 5eccbd62fec8..05d25e645b46 100644
> >> --- a/arch/arm64/kernel/hyp-stub.S
> >> +++ b/arch/arm64/kernel/hyp-stub.S
> >> @@ -115,9 +115,10 @@ SYM_CODE_START_LOCAL(mutate_to_vhe)
> >>   	mrs_s	x0, SYS_VBAR_EL12
> >>   	msr	vbar_el1, x0
> >>   -	// Use EL2 translations for SPE and disable access from EL1
> >> +	// Use EL2 translations for SPE & TRBE and disable access from EL1
> >>   	mrs	x0, mdcr_el2
> >>   	bic	x0, x0, #(MDCR_EL2_E2PB_MASK << MDCR_EL2_E2PB_SHIFT)
> >> +	bic	x0, x0, #(MDCR_EL2_E2TB_MASK << MDCR_EL2_E2TB_SHIFT)
> >>   	msr	mdcr_el2, x0
> >>     	// Transfer the MM state from EL1 to EL2
> >> diff --git a/arch/arm64/kvm/debug.c b/arch/arm64/kvm/debug.c
> >> index dbc890511631..7b16f42d39f4 100644
> >> --- a/arch/arm64/kvm/debug.c
> >> +++ b/arch/arm64/kvm/debug.c
> >> @@ -89,7 +89,7 @@ void kvm_arm_reset_debug_ptr(struct kvm_vcpu *vcpu)
> >>    *  - Debug ROM Address (MDCR_EL2_TDRA)
> >>    *  - OS related registers (MDCR_EL2_TDOSA)
> >>    *  - Statistical profiler (MDCR_EL2_TPMS/MDCR_EL2_E2PB)
> >> - *  - Self-hosted Trace Filter controls (MDCR_EL2_TTRF)
> >> + *  - Self-hosted Trace (MDCR_EL2_TTRF/MDCR_EL2_E2TB)
> > 
> > For the record, this is likely to conflict with [1], although that
> > patch still has some issues.
> 
> Thanks for the heads up. I think that patch will also conflict with my
> fixes that is queued in kvmarm/fixes.

Most probably. This is a popular landing spot, these days...

> 
> 
> > 
> >>    *
> >>    * Additionally, KVM only traps guest accesses to the debug registers if
> >>    * the guest is not actively using them (see the KVM_ARM64_DEBUG_DIRTY
> >> @@ -107,8 +107,8 @@ void kvm_arm_setup_debug(struct kvm_vcpu *vcpu)
> >>   	trace_kvm_arm_setup_debug(vcpu, vcpu->guest_debug);
> >>     	/*
> >> -	 * This also clears MDCR_EL2_E2PB_MASK to disable guest access
> >> -	 * to the profiling buffer.
> >> +	 * This also clears MDCR_EL2_E2PB_MASK and MDCR_EL2_E2TB_MASK
> >> +	 * to disable guest access to the profiling and trace buffers
> >>   	 */
> >>   	vcpu->arch.mdcr_el2 = __this_cpu_read(mdcr_el2) & MDCR_EL2_HPMN_MASK;
> >>   	vcpu->arch.mdcr_el2 |= (MDCR_EL2_TPM |
> >> diff --git a/arch/arm64/kvm/hyp/nvhe/debug-sr.c b/arch/arm64/kvm/hyp/nvhe/debug-sr.c
> >> index f401724f12ef..9499e18dd28f 100644
> >> --- a/arch/arm64/kvm/hyp/nvhe/debug-sr.c
> >> +++ b/arch/arm64/kvm/hyp/nvhe/debug-sr.c
> >> @@ -58,10 +58,51 @@ static void __debug_restore_spe(u64 pmscr_el1)
> >>   	write_sysreg_s(pmscr_el1, SYS_PMSCR_EL1);
> >>   }
> >>   +static void __debug_save_trace(u64 *trfcr_el1)
> >> +{
> >> +
> > 
> > Spurious blank line?
> > 
> 
> Sure, will fix it
> 
> >> +	*trfcr_el1 = 0;
> >> +
> >> +	/* Check if we have TRBE */
> >> +	if (!cpuid_feature_extract_unsigned_field(read_sysreg(id_aa64dfr0_el1),
> >> +						  ID_AA64DFR0_TRBE_SHIFT))
> >> +		return;
> > 
> > Do we have a way to track this that doesn't involve reading an ID
> > register? This is on the hot path, and is going to really suck badly
> > with NV (which traps all ID regs for obvious reasons). I would have
> > hoped that one way or another, we'd have a static key for this.
> 
> TRBE, like SPE can be optionally enabled on a subset of the CPUs. We
> could have a per-CPU static key in the worst case. I guess this would
> apply to SPE as well.

Ah, so you want to support asymmetric tracing... fair enough. But I
don't think you need a per-CPU static key (and I'm not sure how that'd
work either). You could have a static key indicating if *any* CPU
implements tracing, in which case the check only happens when at least
one CPU is capable of tracing.

You would only need a new capability.

> May be we could do this check at kvm_arch_vcpu_load()/put() ?

That would extend the tracing blackout period enormously, wouldn't it?
I'm not sure that's the best thing to do...

> > 
> >> +
> >> +	/* Check we can access the TRBE */
> >> +	if ((read_sysreg_s(SYS_TRBIDR_EL1) & TRBIDR_PROG))
> >> +		return;
> >> +
> >> +	/* Check if the TRBE is enabled */
> >> +	if (!(read_sysreg_s(SYS_TRBLIMITR_EL1) & TRBLIMITR_ENABLE))
> >> +		return;
> >> +	/*
> >> +	 * Prohibit trace generation while we are in guest.
> >> +	 * Since access to TRFCR_EL1 is trapped, the guest can't
> >> +	 * modify the filtering set by the host.
> > 
> > If TRFCR_EL1 is trapped, where is the trap handling? This series
> > doesn't touch sys_regs.c, so I assume you rely on the "inject an UNDEF
> > for anything unknown" default behaviour?
> 
> Yes.
> 
> > 
> > If that's the case, I'd rather you add an explicit handler.
> > 
> 
> I could add one.

Thanks,

	M.

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