On Wed, Oct 05, 2022 at 11:50:09AM +0200, Eric Auger wrote: > Hi Alexandru, > > On 10/5/22 11:21, Alexandru Elisei wrote: > > Hi Eric, > > > > On Tue, Oct 04, 2022 at 07:31:25PM +0200, Eric Auger wrote: > >> Hi Alexandru, > >> > >> On 10/4/22 18:58, Alexandru Elisei wrote: > >>> Hi Eric, > >>> > >>> On Tue, Oct 04, 2022 at 06:20:23PM +0200, Eric Auger wrote: > >>>> Hi Ricardo, Marc, > >>>> > >>>> On 8/5/22 02:41, Ricardo Koller wrote: > >>>>> There are some tests that fail when running on bare metal (including a > >>>>> passthrough prototype). There are three issues with the tests. The > >>>>> first one is that there are some missing isb()'s between enabling event > >>>>> counting and the actual counting. This wasn't an issue on KVM as > >>>>> trapping on registers served as context synchronization events. The > >>>>> second issue is that some tests assume that registers reset to 0. And > >>>>> finally, the third issue is that overflowing the low counter of a > >>>>> chained event sets the overflow flag in PMVOS and some tests fail by > >>>>> checking for it not being set. > >>>>> > >>>>> Addressed all comments from the previous version: > >>>>> https://lore.kernel.org/kvmarm/20220803182328.2438598-1-ricarkol@xxxxxxxxxx/T/#t > >>>>> - adding missing isb() and fixed the commit message (Alexandru). > >>>>> - fixed wording of a report() check (Andrew). > >>>>> > >>>>> Thanks! > >>>>> Ricardo > >>>>> > >>>>> Ricardo Koller (3): > >>>>> arm: pmu: Add missing isb()'s after sys register writing > >>>>> arm: pmu: Reset the pmu registers before starting some tests > >>>>> arm: pmu: Check for overflow in the low counter in chained counters > >>>>> tests > >>>>> > >>>>> arm/pmu.c | 56 ++++++++++++++++++++++++++++++++++++++----------------- > >>>>> 1 file changed, 39 insertions(+), 17 deletions(-) > >>>>> > >>>> While testing this series and the related '[PATCH 0/9] KVM: arm64: PMU: > >>>> Fixing chained events, and PMUv3p5 support' I noticed I have kvm unit > >>>> test failures on some machines. This does not seem related to those > >>>> series though since I was able to get them without. The failures happen > >>>> on Amberwing machine for instance with the pmu-chain-promotion. > >>>> > >>>> While further investigating I noticed there is a lot of variability on > >>>> the kvm unit test mem_access_loop() count. I can get the counter = 0x1F > >>>> on the first iteration and 0x96 on the subsequent ones for instance. > >>>> While running mem_access_loop(addr, 20, pmu.pmcr_ro | PMU_PMCR_E) I was > >>>> expecting the counter to be close to 20. It is on some HW. > >>>> > >>>> [..] > >>>> > >>>> So I come to the actual question. Can we do any assumption on the > >>>> (virtual) PMU quality/precision? If not, the tests I originally wrote > >>>> are damned to fail on some HW (on some other they always pass) and I > >>>> need to make a decision wrt re-writing part of them, expecially those > >>>> which expect overflow after a given amount of ops. Otherwise, there is > >>>> either something wrong in the test (asm?) or in KVM PMU emulation. I don't think it's the asm because in that case the counter value should be the same every time (even if wrong). > >>>> > >>>> I tried to bisect because I did observe the same behavior on some older > >>>> kernels but the bisect was not successful as the issue does not happen > >>>> always. > >>>> > >>>> Thoughts? > >>> Looking at mem_access_loop(), the first thing that jumps out is the fact > >>> that is missing a DSB barrier. ISB affects only instructions, not memory > >>> accesses and without a DSB, the PE can reorder memory accesses however it > >>> sees fit. > >> Following your suggestion I added a dsh ish at the end of loop and > >> before disabling pmcr_el0 (I hope this is the place you were thinking > >> of) but unfortunately it does not seem to fix my issue. > > Yes, DSB ISH after "b.gt 1b\n" and before the write to PMCR_EL0 that > > disables the PMU. > > > > I think you also need a DSB ISH before the write to PMCR_EL0 that enables > > the PMU in the first instruction of the asm block. In your example, the > > MEM_ACCESS event count is higher than expected, and one explanation for the > > large disparity that I can think of is that previous memory accesses are > > reordered past the instruction that enables the PMU, which makes the PMU > > add these events to the total event count. > > Makes sense. I added those at the 2 locations but unfortunately it does > not change the result for me. > > > >>> I also believe precise_instrs_loop() to be in the same situation, as the > >>> architecture doesn't guarantee that the cycle counter increments after > >>> every CPU cycle (ARM DDI 0487I.a, page D11-5246): > >>> > >>> "Although the architecture requires that direct reads of PMCCNTR_EL0 or > >>> PMCCNTR occur in program order, there is no requirement that the count > >>> increments between two such reads. Even when the counter is incrementing on > >>> every clock cycle, software might need check that the difference between > >>> two reads of the counter is nonzero." > >> OK > >>> There's also an entire section in ARM DDI 0487I.a dedicated to this, titled > >>> "A reasonable degree of inaccuracy" (page D11-5248). I'll post some > >>> snippets that I found interesting, but there are more examples and > >>> explanations to be found in that chapter. > >> yeah I saw that, hence my question about the reasonable disparity we can > >> expect from the HW/SW stack. > >>> "In exceptional circumstances, such as a change in Security state or other > >>> boundary condition, it is acceptable for the count to be inaccurate." > >>> > >>> PMCR writes are trapped by KVM. Is a change in exception level an > >>> "exception circumstance"? Could be, but couldn't find anything definitive. > >>> For example, the architecture allows an implementation to drop an event in > >>> the case of an interrupt: > >>> > >>> "However, dropping a single branch count as the result of a rare > >>> interaction with an interrupt is acceptable." > >>> > >>> So events could definitely be dropped because of an interrupt for the host. > >>> > >>> And there's also this: > >>> > >>> "The imprecision means that the counter might have counted an event around > >>> the time the counter was disabled, but does not allow the event to be > >>> observed as counted after the counter was disabled." > >> In our case there seems to be a huge discrepancy. > > I agree. There is this about the MEM_ACCESS event in the Arm ARM: > > > > "The counter counts each Memory-read operation or Memory-write operation > > that the PE makes." > > > > As for what a Memory-read operation is (emphasis added by me): > > > > "A memory-read operation might be due to: > > The result of an architecturally executed memory-reading instructions. > > The result of a Speculatively executed memory-reading instructions <- this > > is why the DSB ISH is needed before enabling the PMU. > > **A translation table walk**." > > > > Those extra memory accesses might be caused by the table walker deciding to > > walk the tables, speculatively or not. Software has no control over the > > table walker (as long as it is enabled). > That's indeed an interesting track. But can it be possible that for 20 > expected load instructions we end up with ~150 actual memory accesses. > I can't help thinking this is a quite surprising amount. Also the > pattern is surprising: the first iteration gives low counter count (~30) > while subsequent ones bring higher and constant ones (~150). I would > have expected the opposite, no? I will try to run the same experience on > various HW I have access to. > > Anyway there is a problem while interpreting the result of the tests. > Either it can happen on some HW (it is a valid behavior according to the > ARM spec) and the test is simply not runnable or it is a bug somewhere > in the SW stack. > > It would be interesting to run the same tests at baremetal level on > Amberwing and see what are the results. Ricardo/Drew, could you give > some links about the setup? Actually, the "bare metal" tests I performed were on a prototype passthrough implementation: https://github.com/ricarkol/linux/commit/c2b009e813e18e89d6945915bd3ae5787bbe3164 Let me know how it goes. Thanks, Ricardo > > Thanks > > Eric > > > > Thanks, > > Alex > > > >>> If you want my opinion, if it is necessary to count the number of events > >>> for a test instead, I would define a margin of error on the number of > >>> events counted. Or the test could be changed to check that at least one > >>> such event was observed. > >> I agree with you on the fact a reasonable margin must be observed and > >> the tests may need to be rewritten to account for the observed disparity > >> if considered "normal". Another way to proceed is to compute the > >> disparity before launching the main tests and if too big, skip the main > >> tests. Again on some HW, the counts are really 'as expected' and constant. > >> > >> Thanks! > >> > >> Eric > >>> Thanks, > >>> Alex > >>> >
