[RFC PATCH v2 0/2] lib: sbi_pmu: Add PMU snapshot extension

[Inochi Amaoto]

>On Fri, Jan 12, 2024 at 3:22 PM Inochi Amaoto <inochiama at outlook.com> wrote:
>>
>>> On Wed, Jan 10, 2024 at 4:34 PM Inochi Amaoto <inochiama at outlook.com> wrote:
>>>>
>>>> Although PMU snapshot extension is not very useful for normal use,
>>>> it can provide a recoverable snapshot for the PMU system. It is useful
>>>> for KVM VMs, especially for VM snapshot.
>>>>
>>>
>>> Can you please further clarify how adding the PMU snapshot support in
>>> OpenSBI helps the KVM VM snapshot?
>>> During a VM snapshot, the host would have saved the values in the vcpu
>>> structure already.
>>> The PMU snapshot save/restore happens only in SBI calls. Thus, the
>>> host has to invoke the SBI call (in context switch path)
>>> to get an updated copy in the shared memory. PMU snapshot helps in
>>> Virtualization as the host virtualizes the counters which
>>> results in traps for each CSR read access within the guest. However,
>>> the host can directly read the CSR as well.
>>>
>>
>> Yes, I have already notice the vcpu have saved these values. IIRC, only
>> `SBI_PMU_START_FLAG_INIT_FROM_SNAPSHOT` can restore the interrupt state.
>> This is not useful for the normal run, but may be required when needing
>> a persistent snapshot, (migration and VM snapshot). As a result, the host
>> should only call this when saving/restoring the VM to persistent storage,
>> and should not call this in the normal run for performance issue.
>>
>>> Adding PMU snapshot in OpenSBI may affect negatively as well depending
>>> on the state of the L1 cache and the access latency.
>>> CSR access should be quite fast in most of the implementations. If it
>>> results in a miss, the latency would be even higher as you have to
>>> fetch the data from L2 or DRAM.
>>>
>>
>> I have already notice this latency problem. As I mention before, this
>> should be called by the hypervisor when vcpu init/exit. So this problem
>
>Why would a hypervisor call counter start/stop calls during the vcpu
>init/exit path ?
>These are PMU specific snapshots for counters that are in use already.
>Every time there is a context switch
>of a vcpu, the perf subsystem in the host would collect the counter
>value anyways.
>During the VM snapshot, those values would be used. The SBI
>implementation should save the counters
>specified in the counter mask specified in the stop call. If a PMU is
>not in use, it wouldn't matter.
>
>> may be acceptable. In fact, I found that we can directly implement the
>> vcpu PMU sampling by pass a page for guset to host SBI. But I struggle on
>
>Not sure what do you mean by that. vcpu PMU sampling support patches
>can be found here here[1]
>
>[1] https://lore.kernel.org/lkml/20240110231359.1239367-2-atishp@rivosinc.com/T/#m9943791909628f90ede01303def7f647db81bf9d
>

Thanks for your patch. It seems I have mistaken the PMU OF interrupt for
KVM. It seems that there is no need to delegate the interrupt to SBI when
restoring the VM snapshot, and we can process them all at HS-mode, Right?