[PATCH] perf/amd/ibs: Improve guest profiling

[Ravi Bangoria]

IBS captures the RIP but not its privilege level. Since the NMI is
delivered with delay, CPL can change between the IBS tag and NMI
delivery. Add a check to catch and discard invalid guest samples
using CPL stored in vCPU save area. This will work when there is
user/kernel CPL change in between IBS tag and NMI delivery within
the guest boundary. But it won't work when there is a guest entry
or exit in between IBS tag and NMI delivery.

When profiling a guest and the IBS RIP is valid, assign the sample
IP from the IBS-captured RIP and set PERF_SAMPLE_IP in sample_flags
so that perf_prepare_sample() do not overwrite the RIP with
perf_guest_get_ip() from the vCPU save area. This keeps the perf
sample IP consistent with IBS raw data, data_src, weight, phy_addr
etc. The privilege level in the perf "misc" field can now go out
of sync, as it is taken from the vCPU save area.

Reported-by: Ian Rogers <irogers@xxxxxxxxxx>
Closes: https://lore.kernel.org/r/CAP-5=fV_cJskvLRZhQQXMGAcPUb_Rg_b30PDJNXzxL49JK4B5g@xxxxxxxxxxxxxx
Signed-off-by: Ravi Bangoria <ravi.bangoria@xxxxxxx>
---
 arch/x86/events/amd/ibs.c | 37 +++++++++++++++++++++++++++++++++++++
 1 file changed, 37 insertions(+)

diff --git a/arch/x86/events/amd/ibs.c b/arch/x86/events/amd/ibs.c
index eeb607b84dda..70408b0b1597 100644
--- a/arch/x86/events/amd/ibs.c
+++ b/arch/x86/events/amd/ibs.c
@@ -1415,6 +1415,7 @@ static int perf_ibs_handle_irq(struct perf_ibs *perf_ibs, struct pt_regs *iregs)
 	unsigned int msr;
 	u64 *buf, *config, period, new_config = 0;
 	int br_target_idx = -1;
+	unsigned int guest_state;
 
 	if (!test_bit(IBS_STARTED, pcpu->state)) {
 fail:
@@ -1526,6 +1527,42 @@ static int perf_ibs_handle_irq(struct perf_ibs *perf_ibs, struct pt_regs *iregs)
 		regs.flags |= PERF_EFLAGS_EXACT;
 	}
 
+	guest_state = perf_guest_state();
+	if (!event->attr.exclude_guest && guest_state & PERF_GUEST_ACTIVE) {
+		/*
+		 * IBS captures the RIP but not its privilege level. Since
+		 * NMI arrives delayed, CPL might change in between IBS tag
+		 * and the NMI delivery. Below checks can identify and filter
+		 * out invalid samples when the CPL changes are within the
+		 * guest boundary. However, these checks fail to handle cases
+		 * where the CPU performs a guest entry or exit in between
+		 * the IBS tag and the NMI delivery.
+		 */
+		if (event->attr.exclude_kernel && !(guest_state & PERF_GUEST_USER)) {
+			throttle = perf_event_account_interrupt(event);
+			goto out;
+		}
+		if (event->attr.exclude_user && guest_state & PERF_GUEST_USER) {
+			throttle = perf_event_account_interrupt(event);
+			goto out;
+		}
+
+		/*
+		 * Assign the IBS RIP value directly in the perf sample here
+		 * to prevent perf_prepare_sample() from retrieving it from
+		 * the vCPU save-area. With this, rest of the perf sample
+		 * fields (raw data, data_src, weight, phy_addr, etc.) will
+		 * remain in sync with sample IP. However, privilege level
+		 * captured as part of perf sample "misc" field could now
+		 * go out of sync since privilege level is fetched from the
+		 * vCPU save area.
+		 */
+		if (regs.flags & PERF_EFLAGS_EXACT) {
+			data.ip = regs.ip;
+			data.sample_flags |= PERF_SAMPLE_IP;
+		}
+	}
+
 	if (((ibs_caps & IBS_CAPS_BIT63_FILTER) ||
 	     (event->attr.config2 & IBS_SW_FILTER_MASK)) &&
 	    perf_ibs_discard_sample(perf_ibs, event, &regs, &ibs_data, br_target_idx)) {
-- 
2.43.0