powerpc/64s/perf: Always use SIAR for kernel interrupts

If an interrupt is taken in kernel mode, always use SIAR for it rather than
looking at regs_sipr. This prevents samples piling up around interrupt
enable (hard enable or interrupt replay via soft enable) in PMUs / modes
where the PR sample indication is not in synch with SIAR.

This results in better sampling of interrupt entry and exit in particular.
Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Tested-by: Athira Rajeev <atrajeev@linux.vnet.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20210720141504.420110-1-npiggin@gmail.com

arch/powerpc/perf/core-book3s.c

@@ -340,6 +340,13 @@ static inline void perf_read_regs(struct pt_regs *regs)
 	 * If the PMU doesn't update the SIAR for non marked events use
 	 * pt_regs.
 	 *
+	 * If regs is a kernel interrupt, always use SIAR. Some PMUs have an
+	 * issue with regs_sipr not being in synch with SIAR in interrupt entry
+	 * and return sequences, which can result in regs_sipr being true for
+	 * kernel interrupts and SIAR, which has the effect of causing samples
+	 * to pile up at mtmsrd MSR[EE] 0->1 or pending irq replay around
+	 * interrupt entry/exit.
+	 *
 	 * If the PMU has HV/PR flags then check to see if they
 	 * place the exception in userspace. If so, use pt_regs. In
 	 * continuous sampling mode the SIAR and the PMU exception are
@@ -356,6 +363,8 @@ static inline void perf_read_regs(struct pt_regs *regs)
 		use_siar = 1;
 	else if ((ppmu->flags & PPMU_NO_CONT_SAMPLING))
 		use_siar = 0;
+	else if (!user_mode(regs))
+		use_siar = 1;
 	else if (!(ppmu->flags & PPMU_NO_SIPR) && regs_sipr(regs))
 		use_siar = 0;
 	else