[PATCH 17/17] x86: retpolines: eliminate retpoline from msr event handlers

Andrea Arcangeli
2019-09-20T21:26:08Z

It's enough to check the value and issue the direct call.

After this commit is applied, here the most common retpolines executed
under a high resolution timer workload in the guest on a VMX host:

[..]
@[
trace_retpoline+1
__trace_retpoline+30
__x86_indirect_thunk_rax+33
do_syscall_64+89
entry_SYSCALL_64_after_hwframe+68
]: 267
@[]: 2256
@[
trace_retpoline+1
__trace_retpoline+30
__x86_indirect_thunk_rax+33
__kvm_wait_lapic_expire+284
vmx_vcpu_run.part.97+1091
vcpu_enter_guest+377
kvm_arch_vcpu_ioctl_run+261
kvm_vcpu_ioctl+559
do_vfs_ioctl+164
ksys_ioctl+96
__x64_sys_ioctl+22
do_syscall_64+89
entry_SYSCALL_64_after_hwframe+68
]: 2390
@[]: 33410

@total: 315707

Note the highest hit above is __delay so probably not worth optimizing
even if it would be more frequent than 2k hits per sec.

Signed-off-by: Andrea Arcangeli <aarcange@redhat.com>
---
arch/x86/events/intel/core.c | 11 +++++++++++
1 file changed, 11 insertions(+)

diff --git a/arch/x86/events/intel/core.c b/arch/x86/events/intel/core.c
index 27ee47a7be66..65b383d5e062 100644
--- a/arch/x86/events/intel/core.c
+++ b/arch/x86/events/intel/core.c
@@ -3323,8 +3323,19 @@ static int intel_pmu_hw_config(struct perf_event *event)
return 0;
}

+#ifdef CONFIG_RETPOLINE
+static struct perf_guest_switch_msr *core_guest_get_msrs(int *nr);
+static struct perf_guest_switch_msr *intel_guest_get_msrs(int *nr);
+#endif
+
struct perf_guest_switch_msr *perf_guest_get_msrs(int *nr)
{
+#ifdef CONFIG_RETPOLINE
+ if (x86_pmu.guest_get_msrs == intel_guest_get_msrs)
+ return intel_guest_get_msrs(nr);
+ else if (x86_pmu.guest_get_msrs == core_guest_get_msrs)
+ return core_guest_get_msrs(nr);
+#endif
if (x86_pmu.guest_get_msrs)
return x86_pmu.guest_get_msrs(nr);
*nr = 0;