s390/cpumf: simplify detection of guest samples

There are three different code levels in regard to the identification
of guest samples. They differ in the way the LPP instruction is used.

1) Old kernels without the LPP instruction. The guest program parameter
is always zero.
2) Newer kernels load the process pid into the program parameter with LPP.
The guest program parameter is non-zero if the guest executes in a
process != idle.
3) The latest kernels load ((1UL << 31) | pid) with LPP to make the value
non-zero even for the idle task. The guest program parameter is non-zero
if the guest is running.

All kernels load the process pid to CR4 on context switch. The CPU sampling
code uses the value in CR4 to decide between guest and host samples in case
the guest program parameter is zero. The three cases:

1) CR4==pid, gpp==0
2) CR4==pid, gpp==pid
3) CR4==pid, gpp==((1UL << 31) | pid)

The load-control instruction to load the pid into CR4 is expensive and the
goal is to remove it. To distinguish the host CR4 from the guest pid for
the idle process the maximum value 0xffff for the PASN is used.
This adds a fourth case for a guest OS with an updated kernel:

4) CR4==0xffff, gpp=((1UL << 31) | pid)

The host kernel will have CR4==0xffff and will use (gpp!=0 || CR4!==0xffff)
to identify guest samples. This works nicely with all 4 cases, the only
possible issue would be a guest with an old kernel (gpp==0) and a process
pid of 0xffff. Well, don't do that..

Suggested-by: Christian Borntraeger <[e-mail]>
Reviewed-by: Christian Borntraeger <[e-mail]>
Signed-off-by: Martin Schwidefsky <[e-mail]>

arch/s390/kernel/entry.S

···
 	stg	%r3,__LC_CURRENT		# store task struct 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 */
-	xc	__SF_EMPTY(8,%r15),__SF_EMPTY(%r15)
-	mvc	__SF_EMPTY+4(4,%r15),__TASK_pid(%r3)
-	lctlg	%c4,%c4,__SF_EMPTY(%r15)	# 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
 	TSTMSK	__LC_MACHINE_FLAGS,MACHINE_FLAG_LPP

arch/s390/kernel/head64.S

···
 	.quad	0			# cr1: primary space segment table
 	.quad	.Lduct			# cr2: dispatchable unit control table
 	.quad	0			# cr3: instruction authorization
-	.quad	0			# cr4: instruction authorization
+	.quad	0xffff			# cr4: instruction authorization
 	.quad	.Lduct			# cr5: primary-aste origin
 	.quad	0			# cr6:	I/O interrupts
 	.quad	0			# cr7:	secondary space segment table

arch/s390/kernel/perf_cpum_sf.c

···
 	 * sample. Some early samples or samples from guests without
 	 * lpp usage would be misaccounted to the host. We use the asn
 	 * value as an addon heuristic to detect most of these guest samples.
-	 * If the value differs from the host hpp value, we assume to be a
-	 * KVM guest.
+	 * If the value differs from 0xffff (the host value), we assume to
+	 * be a KVM guest.
 	 */
 	switch (sfr->basic.CL) {
 	case 1: /* logical partition */
···
 		sde_regs->in_guest = 1;
 		break;
 	default: /* old machine, use heuristics */
-		if (sfr->basic.gpp ||
-		    sfr->basic.prim_asn != (u16)sfr->basic.hpp)
+		if (sfr->basic.gpp || sfr->basic.prim_asn != 0xffff)
 			sde_regs->in_guest = 1;
 		break;
 	}