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authorLinus Torvalds <torvalds@linux-foundation.org>2016-12-13 15:47:02 -0800
committerLinus Torvalds <torvalds@linux-foundation.org>2016-12-13 15:47:02 -0800
commit93173b5bf2841da7e3a9b0cb1312ef5c87251524 (patch)
tree629de2735f465ce0437f8dba85a00f766bbec31c /Documentation/virtual
parent1c59e1edb13d60b97b7b03b332ceed5d967d4227 (diff)
parentf673b5b2a66332da5358def524dbfb3305c76d8c (diff)
Merge tag 'for-linus' of git://git.kernel.org/pub/scm/virt/kvm/kvm
Pull KVM updates from Paolo Bonzini: "Small release, the most interesting stuff is x86 nested virt improvements. x86: - userspace can now hide nested VMX features from guests - nested VMX can now run Hyper-V in a guest - support for AVX512_4VNNIW and AVX512_FMAPS in KVM - infrastructure support for virtual Intel GPUs. PPC: - support for KVM guests on POWER9 - improved support for interrupt polling - optimizations and cleanups. s390: - two small optimizations, more stuff is in flight and will be in 4.11. ARM: - support for the GICv3 ITS on 32bit platforms" * tag 'for-linus' of git://git.kernel.org/pub/scm/virt/kvm/kvm: (94 commits) arm64: KVM: pmu: Reset PMSELR_EL0.SEL to a sane value before entering the guest KVM: arm/arm64: timer: Check for properly initialized timer on init KVM: arm/arm64: vgic-v2: Limit ITARGETSR bits to number of VCPUs KVM: x86: Handle the kthread worker using the new API KVM: nVMX: invvpid handling improvements KVM: nVMX: check host CR3 on vmentry and vmexit KVM: nVMX: introduce nested_vmx_load_cr3 and call it on vmentry KVM: nVMX: propagate errors from prepare_vmcs02 KVM: nVMX: fix CR3 load if L2 uses PAE paging and EPT KVM: nVMX: load GUEST_EFER after GUEST_CR0 during emulated VM-entry KVM: nVMX: generate MSR_IA32_CR{0,4}_FIXED1 from guest CPUID KVM: nVMX: fix checks on CR{0,4} during virtual VMX operation KVM: nVMX: support restore of VMX capability MSRs KVM: nVMX: generate non-true VMX MSRs based on true versions KVM: x86: Do not clear RFLAGS.TF when a singlestep trap occurs. KVM: x86: Add kvm_skip_emulated_instruction and use it. KVM: VMX: Move skip_emulated_instruction out of nested_vmx_check_vmcs12 KVM: VMX: Reorder some skip_emulated_instruction calls KVM: x86: Add a return value to kvm_emulate_cpuid KVM: PPC: Book3S: Move prototypes for KVM functions into kvm_ppc.h ...
Diffstat (limited to 'Documentation/virtual')
-rw-r--r--Documentation/virtual/kvm/00-INDEX2
-rw-r--r--Documentation/virtual/kvm/api.txt5
-rw-r--r--Documentation/virtual/kvm/halt-polling.txt127
3 files changed, 133 insertions, 1 deletions
diff --git a/Documentation/virtual/kvm/00-INDEX b/Documentation/virtual/kvm/00-INDEX
index fee9f2bf9c64..69fe1a8b7ad1 100644
--- a/Documentation/virtual/kvm/00-INDEX
+++ b/Documentation/virtual/kvm/00-INDEX
@@ -6,6 +6,8 @@ cpuid.txt
- KVM-specific cpuid leaves (x86).
devices/
- KVM_CAP_DEVICE_CTRL userspace API.
+halt-polling.txt
+ - notes on halt-polling
hypercalls.txt
- KVM hypercalls.
locking.txt
diff --git a/Documentation/virtual/kvm/api.txt b/Documentation/virtual/kvm/api.txt
index 6bbceb9a3a19..03145b7cafaa 100644
--- a/Documentation/virtual/kvm/api.txt
+++ b/Documentation/virtual/kvm/api.txt
@@ -2034,6 +2034,8 @@ registers, find a list below:
PPC | KVM_REG_PPC_WORT | 64
PPC | KVM_REG_PPC_SPRG9 | 64
PPC | KVM_REG_PPC_DBSR | 32
+ PPC | KVM_REG_PPC_TIDR | 64
+ PPC | KVM_REG_PPC_PSSCR | 64
PPC | KVM_REG_PPC_TM_GPR0 | 64
...
PPC | KVM_REG_PPC_TM_GPR31 | 64
@@ -2050,6 +2052,7 @@ registers, find a list below:
PPC | KVM_REG_PPC_TM_VSCR | 32
PPC | KVM_REG_PPC_TM_DSCR | 64
PPC | KVM_REG_PPC_TM_TAR | 64
+ PPC | KVM_REG_PPC_TM_XER | 64
| |
MIPS | KVM_REG_MIPS_R0 | 64
...
@@ -2209,7 +2212,7 @@ after pausing the vcpu, but before it is resumed.
4.71 KVM_SIGNAL_MSI
Capability: KVM_CAP_SIGNAL_MSI
-Architectures: x86 arm64
+Architectures: x86 arm arm64
Type: vm ioctl
Parameters: struct kvm_msi (in)
Returns: >0 on delivery, 0 if guest blocked the MSI, and -1 on error
diff --git a/Documentation/virtual/kvm/halt-polling.txt b/Documentation/virtual/kvm/halt-polling.txt
new file mode 100644
index 000000000000..4a8418318769
--- /dev/null
+++ b/Documentation/virtual/kvm/halt-polling.txt
@@ -0,0 +1,127 @@
+The KVM halt polling system
+===========================
+
+The KVM halt polling system provides a feature within KVM whereby the latency
+of a guest can, under some circumstances, be reduced by polling in the host
+for some time period after the guest has elected to no longer run by cedeing.
+That is, when a guest vcpu has ceded, or in the case of powerpc when all of the
+vcpus of a single vcore have ceded, the host kernel polls for wakeup conditions
+before giving up the cpu to the scheduler in order to let something else run.
+
+Polling provides a latency advantage in cases where the guest can be run again
+very quickly by at least saving us a trip through the scheduler, normally on
+the order of a few micro-seconds, although performance benefits are workload
+dependant. In the event that no wakeup source arrives during the polling
+interval or some other task on the runqueue is runnable the scheduler is
+invoked. Thus halt polling is especially useful on workloads with very short
+wakeup periods where the time spent halt polling is minimised and the time
+savings of not invoking the scheduler are distinguishable.
+
+The generic halt polling code is implemented in:
+
+ virt/kvm/kvm_main.c: kvm_vcpu_block()
+
+The powerpc kvm-hv specific case is implemented in:
+
+ arch/powerpc/kvm/book3s_hv.c: kvmppc_vcore_blocked()
+
+Halt Polling Interval
+=====================
+
+The maximum time for which to poll before invoking the scheduler, referred to
+as the halt polling interval, is increased and decreased based on the perceived
+effectiveness of the polling in an attempt to limit pointless polling.
+This value is stored in either the vcpu struct:
+
+ kvm_vcpu->halt_poll_ns
+
+or in the case of powerpc kvm-hv, in the vcore struct:
+
+ kvmppc_vcore->halt_poll_ns
+
+Thus this is a per vcpu (or vcore) value.
+
+During polling if a wakeup source is received within the halt polling interval,
+the interval is left unchanged. In the event that a wakeup source isn't
+received during the polling interval (and thus schedule is invoked) there are
+two options, either the polling interval and total block time[0] were less than
+the global max polling interval (see module params below), or the total block
+time was greater than the global max polling interval.
+
+In the event that both the polling interval and total block time were less than
+the global max polling interval then the polling interval can be increased in
+the hope that next time during the longer polling interval the wake up source
+will be received while the host is polling and the latency benefits will be
+received. The polling interval is grown in the function grow_halt_poll_ns() and
+is multiplied by the module parameter halt_poll_ns_grow.
+
+In the event that the total block time was greater than the global max polling
+interval then the host will never poll for long enough (limited by the global
+max) to wakeup during the polling interval so it may as well be shrunk in order
+to avoid pointless polling. The polling interval is shrunk in the function
+shrink_halt_poll_ns() and is divided by the module parameter
+halt_poll_ns_shrink, or set to 0 iff halt_poll_ns_shrink == 0.
+
+It is worth noting that this adjustment process attempts to hone in on some
+steady state polling interval but will only really do a good job for wakeups
+which come at an approximately constant rate, otherwise there will be constant
+adjustment of the polling interval.
+
+[0] total block time: the time between when the halt polling function is
+ invoked and a wakeup source received (irrespective of
+ whether the scheduler is invoked within that function).
+
+Module Parameters
+=================
+
+The kvm module has 3 tuneable module parameters to adjust the global max
+polling interval as well as the rate at which the polling interval is grown and
+shrunk. These variables are defined in include/linux/kvm_host.h and as module
+parameters in virt/kvm/kvm_main.c, or arch/powerpc/kvm/book3s_hv.c in the
+powerpc kvm-hv case.
+
+Module Parameter | Description | Default Value
+--------------------------------------------------------------------------------
+halt_poll_ns | The global max polling interval | KVM_HALT_POLL_NS_DEFAULT
+ | which defines the ceiling value |
+ | of the polling interval for | (per arch value)
+ | each vcpu. |
+--------------------------------------------------------------------------------
+halt_poll_ns_grow | The value by which the halt | 2
+ | polling interval is multiplied |
+ | in the grow_halt_poll_ns() |
+ | function. |
+--------------------------------------------------------------------------------
+halt_poll_ns_shrink | The value by which the halt | 0
+ | polling interval is divided in |
+ | the shrink_halt_poll_ns() |
+ | function. |
+--------------------------------------------------------------------------------
+
+These module parameters can be set from the debugfs files in:
+
+ /sys/module/kvm/parameters/
+
+Note: that these module parameters are system wide values and are not able to
+ be tuned on a per vm basis.
+
+Further Notes
+=============
+
+- Care should be taken when setting the halt_poll_ns module parameter as a
+large value has the potential to drive the cpu usage to 100% on a machine which
+would be almost entirely idle otherwise. This is because even if a guest has
+wakeups during which very little work is done and which are quite far apart, if
+the period is shorter than the global max polling interval (halt_poll_ns) then
+the host will always poll for the entire block time and thus cpu utilisation
+will go to 100%.
+
+- Halt polling essentially presents a trade off between power usage and latency
+and the module parameters should be used to tune the affinity for this. Idle
+cpu time is essentially converted to host kernel time with the aim of decreasing
+latency when entering the guest.
+
+- Halt polling will only be conducted by the host when no other tasks are
+runnable on that cpu, otherwise the polling will cease immediately and
+schedule will be invoked to allow that other task to run. Thus this doesn't
+allow a guest to denial of service the cpu.