On Thu, Apr 20, 2023, Jinrong Liang wrote:
From: Jinrong Liang cloudliang@tencent.com
From: Jinrong Liang cloudliang@tencent.com
Update the documentation for the KVM_SET_PMU_EVENT_FILTER ioctl to include a detailed description of how fixed performance events are handled in the pmu filter. The action and fixed_counter_bitmap members of the pmu filter to determine whether fixed performance events can be programmed by the guest. This information is helpful for correctly configuring the fixed_counter_bitmap and action fields to filter fixed performance events.
Suggested-by: Like Xu likexu@tencent.com Reported-by: kernel test robot lkp@intel.com Link: https://lore.kernel.org/oe-kbuild-all/202304150850.rx4UDDsB-lkp@intel.com Signed-off-by: Jinrong Liang cloudliang@tencent.com
Please post this separately from the selftests changes.
1 file changed, 21 insertions(+)
diff --git a/Documentation/virt/kvm/api.rst b/Documentation/virt/kvm/api.rst index a69e91088d76..b5836767e0e7 100644 --- a/Documentation/virt/kvm/api.rst +++ b/Documentation/virt/kvm/api.rst @@ -5122,6 +5122,27 @@ Valid values for 'action':: #define KVM_PMU_EVENT_ALLOW 0 #define KVM_PMU_EVENT_DENY 1 +Via this API, KVM userspace can also control the behavior of the VM's fixed +counters (if any) by configuring the "action" and "fixed_counter_bitmap" fields.
+Specifically, KVM follows the following pseudo-code when determining whether to +allow the guest FixCtr[i] to count its pre-defined fixed event::
- FixCtr[i]_is_allowed = (action == ALLOW) && (bitmap & BIT(i)) ||
- (action == DENY) && !(bitmap & BIT(i));
- FixCtr[i]_is_denied = !FixCtr[i]_is_allowed;
+Note once this API interface is called, the default zero value of the field
No, there is no "default" value. Userspace provides the exact value. The KVM *selftest* clears fixed_counter_bitmap in all cases, but there is no default anywhere.
+"fixed_counter_bitmap" will implicitly affect all fixed counters, even if it's
There is no implicit behavior, userspace very explicitly provides fixed_counter_bitmap.
+expected to be used only to control the events on generic counters.
I would rather phrase this as:
--- KVM always consumes fixed_counter_bitmap, it's userspace's responsibility to ensure fixed_counter_bitmap is set correctly, e.g. if userspace wants to define a filter that only affects general purpose counters. ---
+In addition, pre-defined performance events on the fixed counters already have +event_select and unit_mask values defined, which means userspace can also +control fixed counters by configuring "action"+ "events" fields.
+When there is a contradiction between these two polices, the fixed performance +counter will only follow the rule of the pseudo-code above.
This is unnecessary vague. I think what you're saying is, with a slight reword of the first paragraph too:
--- Note, the "events" field also applies to fixed counters' hardcoded event_select and unit_mask values. "fixed_counter_bitmap" has higher priority than "events" if there is a contradiction between the two. ---