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>

diff --git a/arch/s390/kernel/entry.S b/arch/s390/kernel/entry.S
index cd5a191..4ba688c 100644 (file)
--- a/arch/s390/kernel/entry.S
+++ b/arch/s390/kernel/entry.S
@@ -186,6 +186,7 @@ ENTRY(__switch_to)
        stg     %r5,__LC_THREAD_INFO            # store thread info of next
        stg     %r15,__LC_KERNEL_STACK          # store end of kernel stack
        lg      %r15,__THREAD_ksp(%r1)          # load kernel stack of next
+       /* c4 is used in guest detection: arch/s390/kernel/perf_cpum_sf.c */
        lctl    %c4,%c4,__TASK_pid(%r3)         # load pid to control reg. 4
        mvc     __LC_CURRENT_PID(4,%r0),__TASK_pid(%r3) # store pid of next
        lmg     %r6,%r15,__SF_GPRS(%r15)        # load gprs of next task

diff --git a/arch/s390/kernel/perf_cpum_sf.c b/arch/s390/kernel/perf_cpum_sf.c
index 3d8da1e..1a43474 100644 (file)
--- 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;