Re: [RFC PATCH 2/3] arm64: KVM: Support exclude_guest for Coresight trace in nVHE

On 08/08/2023 12:04, Marc Zyngier wrote:

On Fri, 04 Aug 2023 11:13:12 +0100,
James Clark [off-list ref] wrote:

quoted

Currently trace will always be generated in nVHE as long as TRBE isn't
being used. To allow filtering out guest trace, re-apply the filter
rules before switching to the guest.

The TRFCR restore function remains the same.

Signed-off-by: James Clark <redacted>
---
 arch/arm64/kvm/debug.c             |  7 ++++
 arch/arm64/kvm/hyp/nvhe/debug-sr.c | 56 +++++++++++++++++++++++++++---
 2 files changed, 59 insertions(+), 4 deletions(-)

diff --git a/arch/arm64/kvm/debug.c b/arch/arm64/kvm/debug.c
index 8725291cb00a..ebb4db20a859 100644
--- a/arch/arm64/kvm/debug.c
+++ b/arch/arm64/kvm/debug.c

@@ -335,10 +335,17 @@ void kvm_arch_vcpu_load_debug_state_flags(struct kvm_vcpu *vcpu)
 	if (cpuid_feature_extract_unsigned_field(dfr0, ID_AA64DFR0_EL1_TraceBuffer_SHIFT) &&
 	    !(read_sysreg_s(SYS_TRBIDR_EL1) & TRBIDR_EL1_P))
 		vcpu_set_flag(vcpu, DEBUG_STATE_SAVE_TRBE);
+	/*
+	 * Save TRFCR on nVHE if FEAT_TRF exists. This will be done in cases
+	 * where DEBUG_STATE_SAVE_TRBE doesn't completely disable trace.
+	 */
+	if (cpuid_feature_extract_unsigned_field(dfr0, ID_AA64DFR0_EL1_TraceFilt_SHIFT))
+		vcpu_set_flag(vcpu, DEBUG_STATE_SAVE_TRFCR);
 }
 
 void kvm_arch_vcpu_put_debug_state_flags(struct kvm_vcpu *vcpu)
 {
 	vcpu_clear_flag(vcpu, DEBUG_STATE_SAVE_SPE);
 	vcpu_clear_flag(vcpu, DEBUG_STATE_SAVE_TRBE);
+	vcpu_clear_flag(vcpu, DEBUG_STATE_SAVE_TRFCR);
 }

diff --git a/arch/arm64/kvm/hyp/nvhe/debug-sr.c b/arch/arm64/kvm/hyp/nvhe/debug-sr.c
index 4558c02eb352..0e8c85b29b92 100644
--- a/arch/arm64/kvm/hyp/nvhe/debug-sr.c
+++ b/arch/arm64/kvm/hyp/nvhe/debug-sr.c

@@ -51,13 +51,17 @@ static void __debug_restore_spe(u64 pmscr_el1)
 	write_sysreg_s(pmscr_el1, SYS_PMSCR_EL1);
 }
 
-static void __debug_save_trace(u64 *trfcr_el1)
+/*
+ * Save TRFCR and disable trace completely if TRBE is being used. Return true
+ * if trace was disabled.
+ */
+static bool __debug_save_trace(u64 *trfcr_el1)
 {
 	*trfcr_el1 = 0;
 
 	/* Check if the TRBE is enabled */
 	if (!(read_sysreg_s(SYS_TRBLIMITR_EL1) & TRBLIMITR_EL1_E))
-		return;
+		return false;

While you're refactoring this code, please move the zeroing of
*trfcr_el1 under the if statement.

quoted

 	/*
 	 * Prohibit trace generation while we are in guest.
 	 * Since access to TRFCR_EL1 is trapped, the guest can't

@@ -68,6 +72,8 @@ static void __debug_save_trace(u64 *trfcr_el1)
 	isb();
 	/* Drain the trace buffer to memory */
 	tsb_csync();
+
+	return true;
 }
 
 static void __debug_restore_trace(u64 trfcr_el1)

@@ -79,14 +85,55 @@ static void __debug_restore_trace(u64 trfcr_el1)
 	write_sysreg_s(trfcr_el1, SYS_TRFCR_EL1);
 }
 
+#if IS_ENABLED(CONFIG_PERF_EVENTS)

As previously stated, just always compile this. There shouldn't be
anything here that's so large that it becomes a candidate for
exclusion. Hell, even the whole of NV+pKVM are permanent features,
even of most people won't use *any* of that.

quoted

+static inline void __debug_save_trfcr(struct kvm_vcpu *vcpu)
+{
+	u64 trfcr;
+	struct kvm_etm_event etm_event = vcpu->arch.host_debug_state.etm_event;
+
+	/* No change if neither are excluded */
+	if (!etm_event.exclude_guest && !etm_event.exclude_host) {
+		/* Zeroing prevents restoring a stale value */
+		vcpu->arch.host_debug_state.trfcr_el1 = 0;

I find this "zero means do nothing" part very odd. I can see it is
already done, but I really dislike this sort of assumption to avoid
writing to a register.

I'd really prefer we track another version of TRFCR_EL1, compare host
and guest, and decide to avoid writing if they are equal. At least, it
would be readable.

And in the end, expressing *everything* in terms of the register would
really help, instead of the exclude_* stuff that has no place in the
low-level arch code.

Yep, I agree with all of the above, I can make these changes for the
next version. I just want to clarify your point about disabling trace
for protected guests when not in debug mode that I asked about in the
review on patch 1.

Thanks,

	M.

_______________________________________________
linux-arm-kernel mailing list
linux-arm-kernel@lists.infradead.org
http://lists.infradead.org/mailman/listinfo/linux-arm-kernel