On Thu, Nov 21, 2024, Nikita Kalyazin wrote: > On 19/11/2024 13:24, Sean Christopherson wrote: > > None of this justifies breaking host-side, non-paravirt async page faults. If a > > vCPU hits a missing page, KVM can schedule out the vCPU and let something else > > run on the pCPU, or enter idle and let the SMT sibling get more cycles, or maybe > > even enter a low enough sleep state to let other cores turbo a wee bit. > > > > I have no objection to disabling host async page faults, e.g. it's probably a net > > negative for 1:1 vCPU:pCPU pinned setups, but such disabling needs an opt-in from > > userspace. > > That's a good point, I didn't think about it. The async work would still > need to execute somewhere in that case (or sleep in GUP until the page is > available). The "async work" is often an I/O operation, e.g. to pull in the page from disk, or over the network from the source. The *CPU* doesn't need to actively do anything for those operations. The I/O is initiated, so the CPU can do something else, or go idle if there's no other work to be done. > If processing the fault synchronously, the vCPU thread can also sleep in the > same way freeing the pCPU for something else, If and only if the vCPU can handle a PV async #PF. E.g. if the guest kernel flat out doesn't support PV async #PF, or the fault happened while the guest was in an incompatible mode, etc. If KVM doesn't do async #PFs of any kind, the vCPU will spin on the fault until the I/O completes and the page is ready. > so the amount of work to be done looks equivalent (please correct me > otherwise). What's the net gain of moving that to an async work in the host > async fault case? "while allowing interrupt delivery into the guest." -- is > this the main advantage?
