[PATCH v7 0/4] KVM: X86: Add Paravirt TLB Shootdown

Wanpeng Li
2017-11-30T06:01:20Z

Remote flushing api's does a busy wait which is fine in bare-metal
scenario. But with-in the guest, the vcpus might have been pre-empted
or blocked. In this scenario, the initator vcpu would end up
busy-waiting for a long amount of time.

This patch set implements para-virt flush tlbs making sure that it
does not wait for vcpus that are sleeping. And all the sleeping vcpus
flush the tlb on guest enter. Idea was discussed here:
https://lkml.org/lkml/2012/2/20/157

The best result is achieved when we're overcommiting the host by running
multiple vCPUs on each pCPU. In this case PV tlb flush avoids touching
vCPUs which are not scheduled and avoid the wait on the main CPU.

In addition, thanks for commit 9e52fc2b50d ("x86/mm: Enable RCU based
page table freeing (CONFIG_HAVE_RCU_TABLE_FREE=y)")

Testing on a Xeon Gold 6142 2.6GHz 2 sockets, 32 cores, 64 threads,
so 64 pCPUs, and each VM is 64 vCPUs.

ebizzy -M
vanilla optimized boost
1VM 46799 48670 4%
2VM 23962 42691 78%
3VM 16152 37539 132%

Note: The patchset is not rebased against "locking/qspinlock/x86: Avoid
test-and-set when PV_DEDICATED is set" v3 since I can still observe a
little improvement for 64 vCPUs on 64 pCPUs, it is due to the system
is not completely isolated, there are many housekeeping tasks work
sporadically, and vCPUs are preemted some times, I also confirm this
when adding some print to the kvm_flush_tlb_others. After PV_DEDICATED
is merged, we can disable pv tlb flush when not overcommiting if it
is needed.

v6 -> v7:
* don't check !flushmask
* use arch_initcall() to achieve late allocate percpu mask

v5 -> v6:
* fix the percpu mask
* rebase against latest kvm/queue

v4 -> v5:
* flushmask instead of cpumask

v3 -> v4:
* use READ_ONCE()
* use try_cmpxchg instead of cmpxchg
* add {} to if
* no FLUSH flags to preserve during set_preempted
* "KVM: X86" prefix to patch subject

v2 -> v3:
* percpu cpumask

v1 -> v2:
* a new CPUID feature bit
* fix cmpxchg check
* use kvm_vcpu_flush_tlb() to get the statistics right
* just OR the KVM_VCPU_PREEMPTED in kvm_steal_time_set_preempted
* add a new bool argument to kvm_x86_ops->tlb_flush
* __cpumask_clear_cpu() instead of cpumask_clear_cpu()
* not put cpumask_t on stack
* rebase the patchset against "locking/qspinlock/x86: Avoid
test-and-set when PV_DEDICATED is set" v3

Wanpeng Li (4):
KVM: X86: Add vCPU running/preempted state
KVM: X86: Add Paravirt TLB Shootdown
KVM: X86: introduce invalidate_gpa argument to tlb flush
KVM: X86: Add flush_on_enter before guest enter

Documentation/virtual/kvm/cpuid.txt | 4 +++
arch/x86/include/asm/kvm_host.h | 2 +-
arch/x86/include/uapi/asm/kvm_para.h | 5 ++++
arch/x86/kernel/kvm.c | 49 +++++++++++++++++++++++++++++++++++-
arch/x86/kvm/cpuid.c | 3 ++-
arch/x86/kvm/svm.c | 14 +++++------
arch/x86/kvm/vmx.c | 21 ++++++++--------
arch/x86/kvm/x86.c | 25 +++++++++++-------
8 files changed, 94 insertions(+), 29 deletions(-)

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2.7.4