On 2021-02-04 12:32, Alexandru Elisei wrote:

Hi,

On 2/3/21 1:28 PM, Marc Zyngier wrote:

quoted

On 2021-02-03 12:39, Auger Eric wrote:

quoted

Hi,

On 2/3/21 12:20 PM, Marc Zyngier wrote:

quoted

On 2021-02-03 11:07, Auger Eric wrote:

quoted

Hi Marc,
On 2/3/21 11:36 AM, Marc Zyngier wrote:

quoted

Hi Eric,

On 2021-01-27 17:53, Auger Eric wrote:

quoted

Hi Marc,

On 1/25/21 1:26 PM, Marc Zyngier wrote:

quoted

Upgrading the PMU code from ARMv8.1 to ARMv8.4 turns out to be
pretty easy. All that is required is support for PMMIR_EL1, 
which
is read-only, and for which returning 0 is a valid option as 
long
as we don't advertise STALL_SLOT as an implemented event.

Let's just do that and adjust what we return to the guest.

Signed-off-by: Marc Zyngier <maz@kernel.org>
---
 arch/arm64/include/asm/sysreg.h |  3 +++
 arch/arm64/kvm/pmu-emul.c       |  6 ++++++
 arch/arm64/kvm/sys_regs.c       | 11 +++++++----
 3 files changed, 16 insertions(+), 4 deletions(-)

diff --git a/arch/arm64/include/asm/sysreg.h

b/arch/arm64/include/asm/sysreg.h
index 8b5e7e5c3cc8..2fb3f386588c 100644

--- a/arch/arm64/include/asm/sysreg.h
+++ b/arch/arm64/include/asm/sysreg.h

@@ -846,7 +846,10 @@

 #define ID_DFR0_PERFMON_SHIFT        24

+#define ID_DFR0_PERFMON_8_0        0x3
 #define ID_DFR0_PERFMON_8_1        0x4
+#define ID_DFR0_PERFMON_8_4        0x5
+#define ID_DFR0_PERFMON_8_5        0x6

 #define ID_ISAR4_SWP_FRAC_SHIFT        28
 #define ID_ISAR4_PSR_M_SHIFT        24

diff --git a/arch/arm64/kvm/pmu-emul.c 

b/arch/arm64/kvm/pmu-emul.c
index 398f6df1bbe4..72cd704a8368 100644

--- a/arch/arm64/kvm/pmu-emul.c
+++ b/arch/arm64/kvm/pmu-emul.c

@@ -795,6 +795,12 @@ u64 kvm_pmu_get_pmceid(struct kvm_vcpu 

*vcpu,
bool pmceid1)
         base = 0;
     } else {
         val = read_sysreg(pmceid1_el0);
+        /*
+         * Don't advertise STALL_SLOT, as PMMIR_EL0 is handled
+         * as RAZ
+         */
+        if (vcpu->kvm->arch.pmuver >= ID_AA64DFR0_PMUVER_8_4)
+            val &= ~BIT_ULL(ARMV8_PMUV3_PERFCTR_STALL_SLOT - 
32);

what about the STALL_SLOT_BACKEND and FRONTEND events then?

Aren't these a mandatory ARMv8.1 feature? I don't see a reason to
drop them.

I understand the 3 are linked together.

In D7.11 it is said
"
When any of the following common events are implemented, all three 
of
them are implemented:
0x003D , STALL_SLOT_BACKEND, No operation sent for execution on a 
Slot
due to the backend,
0x003E , STALL_SLOT_FRONTEND, No operation sent for execution on a 
Slot
due to the frontend.
0x003F , STALL_SLOT, No operation sent for execution on a Slot.
"

They are linked in the sense that they report related events, but 
they
don't have to be implemented in the same level of the architecure, 
if only
because BACKEND/FRONTEND were introducedway before ARMv8.4.

What the architecture says is:

- For FEAT_PMUv3p1 (ARMv8.1):
  "The STALL_FRONTEND and STALL_BACKEND events are required to be
   implemented." (A2.4.1, DDI0487G.a)

OK

quoted

- For FEAT_PMUv3p4 (ARMv8.4):
  "If FEAT_PMUv3p4 is implemented:
   - If STALL_SLOT is not implemented, it is IMPLEMENTATION DEFINED
whether the PMMIR System registers are implemented.
   - If STALL_SLOT is implemented, then the PMMIR System registers 
are
implemented." (D7-2873, DDI0487G.a)

So while BACKEND/FRONTEND are required in an ARMv8.4 implementation
by virtue of being mandatory in ARMv8.1, STALL_SLOT isn't at any 
point.

But then how do you understand "When any of the following common 
events
are implemented, all three of them are implemented"?

I think that's wholly inconsistent, because it would mean that 
STALL_SLOT
isn't optional on ARMv8.4, and would make PMMIR mandatory.

I think there's some confusion regarding the event names. From my 
reading of the
architecture, STALL != STALL_SLOT, STALL_BACKEND != STALL_SLOT_BACKEND,
STALL_FRONTEND != STALL_SLOT_FRONTEND.

Dammit, I hadn't realised that at all. Thanks for putting me right.

STALL{, _BACKEND, _FRONTEND} count the number of CPU cycles where no
instructions
are being executed on the PE (page D7-2872), STALL_SLOT{, _BACKEND, 
_FRONTEND}
count the number of slots where no instructions are being executed
(page D7-2873).

STALL_{BACKEND, FRONTEND} are required by ARMv8.1-PMU (pages A2-76, 
D7-2913);
STALL is required by ARMv8.4-PMU (page D7-2914).

STALL_SLOT{, _BACKEND, _FRONTEND} are optional in ARMv8.4-PMU (pages 
D7-2913,
D7-2914), but if one of them is implemented, all of them must be
implemented (page D7-2914).

The problem I see with this patch is that it doesn't clear the
STALL_SLOT_{BACKEND, FRONTEND} event bits along with the STALL_SLOT bit 
from
PMCEID1_EL0.

OK, so Eric was right, and I'm an illiterate idiot! I'll fix the patch 
and repost
the whole thing.

Thanks,

         M.
-- 
Jazz is not dead. It just smells funny...

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