Notes about PV TLB Shootdown
A TLB is a cache of translation from memory virtual address to physical address. When a CPU changes virtual to physical mapping of an address, it needs to invalidate other CPUs’ stale mapping in their respective caches. This process is called TLB shootdown.
Modern operating systems consider TLB shootdown operations to be performance critical and so optimize them to exhibit very low latency.The implementation of these operations is therefore architected to ensure that shootdowns can be completed with very low latencies through the use of IPI based signalling.
Remote TLB flush does a busy wait which is fine in bare-metal scenario.But within the guest, the vCPUs might have been preempted or blocked. In this scenario, the initiator vCPU would end up busy-waiting for a long amount of time; it also consumes CPU unnecessarily to wake up the target of the shootdown.
Idea:
In PV TLB shootdown, the TLB flush initiator vCPU will not wait the sleeping vCPU, instead it just set a flag in the guest-vmm shared area and then kvm will check this flag and do the TLB flush when the sleeping vCPU come to run.
实现细节也很有意思，感兴趣的读者可以去仔细阅读代码+文末的参考资料，本文就不赘述了。
参考资料: