1. 21 Jan, 2020 1 commit
    • Wanpeng Li's avatar
      KVM: VMX: FIXED+PHYSICAL mode single target IPI fastpath · 1e9e2622
      Wanpeng Li authored
      ICR and TSCDEADLINE MSRs write cause the main MSRs write vmexits in our
      product observation, multicast IPIs are not as common as unicast IPI like
      RESCHEDULE_VECTOR and CALL_FUNCTION_SINGLE_VECTOR etc.
      
      This patch introduce a mechanism to handle certain performance-critical
      WRMSRs in a very early stage of KVM VMExit handler.
      
      This mechanism is specifically used for accelerating writes to x2APIC ICR
      that attempt to send a virtual IPI with physical destination-mode, fixed
      delivery-mode and single target. Which was found as one of the main causes
      of VMExits for Linux workloads.
      
      The reason this mechanism significantly reduce the latency of such virtual
      IPIs is by sending the physical IPI to the target vCPU in a very early stage
      of KVM VMExit handler, before host interrupts are enabled and before expensive
      operations such as reacquiring KVM’s SRCU lock.
      Latency is reduced even more when KVM is able to use APICv posted-interrupt
      mechanism (which allows to deliver the virtual IPI directly to target vCPU
      without the need to kick it to host).
      
      Testing on Xeon Skylake server:
      
      The virtual IPI latency from sender send to receiver receive reduces
      more than 200+ cpu cycles.
      Reviewed-by: default avatarLiran Alon <liran.alon@oracle.com>
      Cc: Paolo Bonzini <pbonzini@redhat.com>
      Cc: Radim Krčmář <rkrcmar@redhat.com>
      Cc: Sean Christopherson <sean.j.christopherson@intel.com>
      Cc: Vitaly Kuznetsov <vkuznets@redhat.com>
      Cc: Liran Alon <liran.alon@oracle.com>
      Signed-off-by: default avatarWanpeng Li <wanpengli@tencent.com>
      Signed-off-by: default avatarPaolo Bonzini <pbonzini@redhat.com>
      1e9e2622
  2. 08 Jan, 2020 38 commits
  3. 05 Jan, 2020 1 commit