s390/cpumf: Improve guest detection heuristics

commit e22cf8ca6f75 ("s390/cpumf: rework program parameter setting
to detect guest samples") requires guest changes to get proper
guest/host. We can do better: We can use the primary asn value,
which is set on all Linux variants to compare this with the host
pp value.
We now have the following cases:
1. Guest using PP
host sample:  gpp == 0, asn == hpp --> host
guest sample: gpp != 0 --> guest
2. Guest not using PP
host sample:  gpp == 0, asn == hpp --> host
guest sample: gpp == 0, asn != hpp --> guest

As soon as the host no longer sets CR4, we must back out
this heuristics - let's add a comment in switch_to.

Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>
Reviewed-by: Hendrik Brueckner <brueckner@linux.vnet.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>

1 files changed, 6 insertions, 3 deletions

diff --git a/arch/s390/kernel/perf_cpum_sf.c b/arch/s390/kernel/perf_cpum_sf.c
index 3d8da1e742c2..1a43474df541 100644
--- a/arch/s390/kernel/perf_cpum_sf.c
+++ b/arch/s390/kernel/perf_cpum_sf.c
@@ -1022,10 +1022,13 @@ static int perf_push_sample(struct perf_event *event, struct sf_raw_sample *sfr)
 	/*
 	 * A non-zero guest program parameter indicates a guest
 	 * sample.
-	 * Note that some early samples might be misaccounted to
-	 * the host.
+	 * Note that some early samples or samples from guests without
+	 * lpp usage would be misaccounted to the host. We use the asn
+	 * value as a heuristic to detect most of these guest samples.
+	 * If the value differs from the host hpp value, we assume
+	 * it to be a KVM guest.
 	 */
-	if (sfr->basic.gpp)
+	if (sfr->basic.gpp || sfr->basic.prim_asn != (u16) sfr->basic.hpp)
 		sde_regs->in_guest = 1;
 
 	overflow = 0;