[PATCH 4/7] KVM: x86/pmu: Add documentation for fixed ctr on PMU filter

[Jinrong Liang <ljr.kernel@gmail.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>
Signed-off-by: Jinrong Liang <cloudliang@tencent.com>
---
 Documentation/virt/kvm/api.rst | 21 +++++++++++++++++++++
 1 file changed, 21 insertions(+)

diff --git a/Documentation/virt/kvm/api.rst b/Documentation/virt/kvm/api.rst
index a69e91088d76..036f5b1a39af 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
+"fixed_counter_bitmap" will implicitly affect all fixed counters, even if it's
+expected to be used only to control the events on generic 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.
+
 4.121 KVM_PPC_SVM_OFF
 ---------------------
 
-- 
2.31.1