arm64/boot: Disable trapping of PMZR_EL0 writes to EL2grafted

__init_el2_fgt2() writes one mask to both HDFGRTR2_EL2 and HDFGWTR2_EL2.
PMZR_EL0 is write-only, so its trap bit, nPMZR_EL0, exists only in
HDFGWTR2_EL2 and is therefore never set: a PMZR_EL0 write from the host
traps to EL2, where the nVHE hypervisor has no handler and BUG()s. The
kernel never writes PMZR_EL0, but kernel.perf_user_access=1 has the PMU
driver set PMUSERENR_EL0.UEN for a task with a user-read event, so a
write from EL0 reaches the trap and takes the host down without a panic
message.

Accumulate the HDFGWTR2_EL2 bits separately, as __init_el2_fgt() already
does for HDFGWTR_EL2, and set nPMZR_EL0 with the other FEAT_PMUv3p9
bits.

Fixes: 858c7bfcb35e1 ("arm64/boot: Enable EL2 requirements for FEAT_PMUv3p9")
Cc: stable@vger.kernel.org
Signed-off-by: Fuad Tabba <fuad.tabba@linux.dev>
Reviewed-by: Anshuman Khandual <anshuman.khandual@arm.com>
Reviewed-by: Oliver Upton <oupton@kernel.org>
Signed-off-by: Will Deacon <will@kernel.org>

1 files changed, 117 insertions, 0 deletions

diff --git a/Documentation/admin-guide/cpu-load.rst b/Documentation/admin-guide/cpu-load.rst
new file mode 100644
index 000000000..21a984337
--- /dev/null
+++ b/Documentation/admin-guide/cpu-load.rst
@@ -0,0 +1,117 @@
+========
+CPU load
+========
+
+Linux exports various bits of information via ``/proc/stat`` and
+``/proc/uptime`` that userland tools, such as top(1), use to calculate
+the average time system spent in a particular state, for example::
+
+    $ iostat
+    Linux 2.6.18.3-exp (linmac)     02/20/2007
+
+    avg-cpu:  %user   %nice %system %iowait  %steal   %idle
+              10.01    0.00    2.92    5.44    0.00   81.63
+
+    ...
+
+Here the system thinks that over the default sampling period the
+system spent 10.01% of the time doing work in user space, 2.92% in the
+kernel, and was overall 81.63% of the time idle.
+
+In most cases the ``/proc/stat``	 information reflects the reality quite
+closely, however due to the nature of how/when the kernel collects
+this data sometimes it can not be trusted at all.
+
+So how is this information collected?  Whenever timer interrupt is
+signalled the kernel looks what kind of task was running at this
+moment and increments the counter that corresponds to this tasks
+kind/state.  The problem with this is that the system could have
+switched between various states multiple times between two timer
+interrupts yet the counter is incremented only for the last state.
+
+
+Example
+-------
+
+If we imagine the system with one task that periodically burns cycles
+in the following manner::
+
+     time line between two timer interrupts
+    |--------------------------------------|
+     ^                                    ^
+     |_ something begins working          |
+                                          |_ something goes to sleep
+                                         (only to be awaken quite soon)
+
+In the above situation the system will be 0% loaded according to the
+``/proc/stat`` (since the timer interrupt will always happen when the
+system is executing the idle handler), but in reality the load is
+closer to 99%.
+
+One can imagine many more situations where this behavior of the kernel
+will lead to quite erratic information inside ``/proc/stat``::
+
+
+	/* gcc -o hog smallhog.c */
+	#include <time.h>
+	#include <limits.h>
+	#include <signal.h>
+	#include <sys/time.h>
+	#define HIST 10
+
+	static volatile sig_atomic_t stop;
+
+	static void sighandler(int signr)
+	{
+		(void) signr;
+		stop = 1;
+	}
+
+	static unsigned long hog (unsigned long niters)
+	{
+		stop = 0;
+		while (!stop && --niters);
+		return niters;
+	}
+
+	int main (void)
+	{
+		int i;
+		struct itimerval it = {
+			.it_interval = { .tv_sec = 0, .tv_usec = 1 },
+			.it_value    = { .tv_sec = 0, .tv_usec = 1 } };
+		sigset_t set;
+		unsigned long v[HIST];
+		double tmp = 0.0;
+		unsigned long n;
+		signal(SIGALRM, &sighandler);
+		setitimer(ITIMER_REAL, &it, NULL);
+
+		hog (ULONG_MAX);
+		for (i = 0; i < HIST; ++i) v[i] = ULONG_MAX - hog(ULONG_MAX);
+		for (i = 0; i < HIST; ++i) tmp += v[i];
+		tmp /= HIST;
+		n = tmp - (tmp / 3.0);
+
+		sigemptyset(&set);
+		sigaddset(&set, SIGALRM);
+
+		for (;;) {
+			hog(n);
+			sigwait(&set, &i);
+		}
+		return 0;
+	}
+
+
+References
+----------
+
+- https://lore.kernel.org/r/loom.20070212T063225-663@post.gmane.org
+- Documentation/filesystems/proc.rst (1.8)
+
+
+Thanks
+------
+
+Con Kolivas, Pavel Machek