[kvm-unit-tests PATCH v3 3/6] arm: pmu: Add extra DSB barriers in the mem_access loop

[Eric Auger <eric.auger@redhat.com>]

The mem access loop currently features ISB barriers only. However
the mem_access loop counts the number of accesses to memory. ISB
do not garantee the PE cannot reorder memory access. Let's
add a DSB ISH before the write to PMCR_EL0 that enables the PMU
to make sure any previous memory access aren't counted in the
loop, another one after the PMU gets enabled (to make sure loop
memory accesses cannot be reordered before the PMU gets enabled)
and a last one after the last iteration, before disabling the PMU.

Signed-off-by: Eric Auger <eric.auger@redhat.com>
Suggested-by: Alexandru Elisei <alexandru.elisei@arm.com>
Reviewed-by: Alexandru Elisei <alexandru.elisei@arm.com>

---
v2 -> v3:
- Added Alexandru's R-b

v1 -> v2:
- added yet another DSB after PMU enabled as suggested by Alexandru

This was discussed in https://lore.kernel.org/all/YzxmHpV2rpfaUdWi@monolith.localdoman/
---
 arm/pmu.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/arm/pmu.c b/arm/pmu.c
index 51c0fe80..74dd4c10 100644
--- a/arm/pmu.c
+++ b/arm/pmu.c
@@ -301,13 +301,16 @@ static void mem_access_loop(void *addr, long loop, uint32_t pmcr)
 {
 	uint64_t pmcr64 = pmcr;
 asm volatile(
+	"       dsb     ish\n"
 	"       msr     pmcr_el0, %[pmcr]\n"
 	"       isb\n"
+	"       dsb     ish\n"
 	"       mov     x10, %[loop]\n"
 	"1:     sub     x10, x10, #1\n"
 	"       ldr	x9, [%[addr]]\n"
 	"       cmp     x10, #0x0\n"
 	"       b.gt    1b\n"
+	"       dsb     ish\n"
 	"       msr     pmcr_el0, xzr\n"
 	"       isb\n"
 	:
-- 
2.38.1