[PATCH 0/2] KVM: Halt-polling documentation cleanups

[PATCH 0/2] KVM: Halt-polling documentation cleanups

[David Matlack]

This series makes some small cleanups to the existing halt-polling
documentation. Patch 1 moves halt-polling.rst out of the x86-specific
directory, and patch 2 clarifies the interaction between
KVM_CAP_HALT_POLL and kvm.halt_poll_ns, which was recently broken and a
source of confusion [1].

[1] https://lore.kernel.org/kvm/03f2f5ab-e809-2ba5-bd98-3393c3b843d2@de.ibm.com/

Cc: Yanan Wang <wangyanan55@huawei.com>
Cc: Christian Borntraeger <borntraeger@linux.ibm.com>

David Matlack (2):
  KVM: Move halt-polling documentation into common directory
  KVM: Document the interaction between KVM_CAP_HALT_POLL and
    halt_poll_ns

 Documentation/virt/kvm/api.rst                    | 15 +++++++--------
 Documentation/virt/kvm/{x86 => }/halt-polling.rst | 13 +++++++++++++
 Documentation/virt/kvm/index.rst                  |  1 +
 Documentation/virt/kvm/x86/index.rst              |  1 -
 4 files changed, 21 insertions(+), 9 deletions(-)
 rename Documentation/virt/kvm/{x86 => }/halt-polling.rst (92%)


base-commit: 7e3bba93f42e9d9abe81344bdba5ddc635b7c449
-- 
2.39.0.rc0.267.gcb52ba06e7-goog

[PATCH 1/2] KVM: Move halt-polling documentation into common directory

[David Matlack]

Move halt-polling.rst into the common KVM documentation directory and
out of the x86-specific directory. Halt-polling is a common feature and
the existing documentation is already written as such.

Signed-off-by: David Matlack <dmatlack@google.com>
---
 Documentation/virt/kvm/{x86 => }/halt-polling.rst | 0
 Documentation/virt/kvm/index.rst                  | 1 +
 Documentation/virt/kvm/x86/index.rst              | 1 -
 3 files changed, 1 insertion(+), 1 deletion(-)
 rename Documentation/virt/kvm/{x86 => }/halt-polling.rst (100%)

diff --git a/Documentation/virt/kvm/x86/halt-polling.rst b/Documentation/virt/kvm/halt-polling.rst
similarity index 100%
rename from Documentation/virt/kvm/x86/halt-polling.rst
rename to Documentation/virt/kvm/halt-polling.rst
diff --git a/Documentation/virt/kvm/index.rst b/Documentation/virt/kvm/index.rst
index e0a2c74e1043..ad13ec55ddfe 100644
--- a/Documentation/virt/kvm/index.rst
+++ b/Documentation/virt/kvm/index.rst
@@ -17,4 +17,5 @@ KVM
 
    locking
    vcpu-requests
+   halt-polling
    review-checklist
diff --git a/Documentation/virt/kvm/x86/index.rst b/Documentation/virt/kvm/x86/index.rst
index 7ff588826b9f..9ece6b8dc817 100644
--- a/Documentation/virt/kvm/x86/index.rst
+++ b/Documentation/virt/kvm/x86/index.rst
@@ -10,7 +10,6 @@ KVM for x86 systems
    amd-memory-encryption
    cpuid
    errata
-   halt-polling
    hypercalls
    mmu
    msr
-- 
2.39.0.rc0.267.gcb52ba06e7-goog

[PATCH 2/2] KVM: Document the interaction between KVM_CAP_HALT_POLL and halt_poll_ns

[David Matlack]

Clarify the existing documentation about how KVM_CAP_HALT_POLL and
halt_poll_ns interact to make it clear that VMs using KVM_CAP_HALT_POLL
ignore halt_poll_ns.

Signed-off-by: David Matlack <dmatlack@google.com>
---
 Documentation/virt/kvm/api.rst          | 15 +++++++--------
 Documentation/virt/kvm/halt-polling.rst | 13 +++++++++++++
 2 files changed, 20 insertions(+), 8 deletions(-)

diff --git a/Documentation/virt/kvm/api.rst b/Documentation/virt/kvm/api.rst
index eee9f857a986..896914e3a847 100644
--- a/Documentation/virt/kvm/api.rst
+++ b/Documentation/virt/kvm/api.rst
@@ -7213,14 +7213,13 @@ veto the transition.
 :Parameters: args[0] is the maximum poll time in nanoseconds
 :Returns: 0 on success; -1 on error
 
-This capability overrides the kvm module parameter halt_poll_ns for the
-target VM.
-
-VCPU polling allows a VCPU to poll for wakeup events instead of immediately
-scheduling during guest halts. The maximum time a VCPU can spend polling is
-controlled by the kvm module parameter halt_poll_ns. This capability allows
-the maximum halt time to specified on a per-VM basis, effectively overriding
-the module parameter for the target VM.
+KVM_CAP_HALT_POLL overrides the kvm.halt_poll_ns module parameter to set the
+maximum halt-polling time for all vCPUs in the target VM. This capability can
+be invoked at any time and any number of times to dynamically change the
+maximum halt-polling time.
+
+See Documentation/virt/kvm/halt-polling.rst for more information on halt
+polling.
 
 7.21 KVM_CAP_X86_USER_SPACE_MSR
 -------------------------------
diff --git a/Documentation/virt/kvm/halt-polling.rst b/Documentation/virt/kvm/halt-polling.rst
index 4922e4a15f18..3fae39b1a5ba 100644
--- a/Documentation/virt/kvm/halt-polling.rst
+++ b/Documentation/virt/kvm/halt-polling.rst
@@ -119,6 +119,19 @@ These module parameters can be set from the debugfs files in:
 Note: that these module parameters are system wide values and are not able to
       be tuned on a per vm basis.
 
+Any changes to these parameters will be picked up by new and existing vCPUs the
+next time they halt, with the notable exception of VMs using KVM_CAP_HALT_POLL
+(see next section).
+
+KVM_CAP_HALT_POLL
+=================
+
+KVM_CAP_HALT_POLL is a VM capability that allows userspace to override halt_poll_ns
+on a per-VM basis. VMs using KVM_CAP_HALT_POLL ignore halt_poll_ns completely (but
+still obey halt_poll_ns_grow, halt_poll_ns_grow_start, and halt_poll_ns_shrink).
+
+See Documentation/virt/kvm/api.rst for more information on this capability.
+
 Further Notes
 =============
 
-- 
2.39.0.rc0.267.gcb52ba06e7-goog

Re: [PATCH 0/2] KVM: Halt-polling documentation cleanups

[Paolo Bonzini]

On 12/1/22 20:52, David Matlack wrote:
> This series makes some small cleanups to the existing halt-polling
> documentation. Patch 1 moves halt-polling.rst out of the x86-specific
> directory, and patch 2 clarifies the interaction between
> KVM_CAP_HALT_POLL and kvm.halt_poll_ns, which was recently broken and a
> source of confusion [1].
> 
> [1] https://lore.kernel.org/kvm/03f2f5ab-e809-2ba5-bd98-3393c3b843d2@de.ibm.com/
> 
> Cc: Yanan Wang <wangyanan55@huawei.com>
> Cc: Christian Borntraeger <borntraeger@linux.ibm.com>
> 
> David Matlack (2):
>    KVM: Move halt-polling documentation into common directory
>    KVM: Document the interaction between KVM_CAP_HALT_POLL and
>      halt_poll_ns
> 
>   Documentation/virt/kvm/api.rst                    | 15 +++++++--------
>   Documentation/virt/kvm/{x86 => }/halt-polling.rst | 13 +++++++++++++
>   Documentation/virt/kvm/index.rst                  |  1 +
>   Documentation/virt/kvm/x86/index.rst              |  1 -
>   4 files changed, 21 insertions(+), 9 deletions(-)
>   rename Documentation/virt/kvm/{x86 => }/halt-polling.rst (92%)
> 
> 
> base-commit: 7e3bba93f42e9d9abe81344bdba5ddc635b7c449

Queued, thanks.

Paolo