On Thu, Sep 12, 2024 at 09:24:40AM -0700, Pawan Gupta wrote: >On Thu, Sep 12, 2024 at 03:44:38PM +0000, Jon Kohler wrote: >> > It is only worth implementing the long sequence in VMEXIT_ONLY mode if it is >> > significantly better than toggling the MSR. >> >> Thanks for the pointer! I hadn’t seen that second sequence. I’ll do measurements on >> three cases and come back with data from an SPR system. >> 1. as-is (wrmsr on entry and exit) >> 2. Short sequence (as a baseline) >> 3. Long sequence > >I wonder if virtual SPEC_CTRL feature introduced in below series can >provide speedup, as it can replace the MSR toggling with faster VMCS >operations: "virtual SPEC_CTRL" won't provide speedup. the wrmsr on entry/exit is still need if guest's (effective) value and host's value are different. "virtual SPEC_CTRL" just prevents guests from toggling some bits. It doesn't switch the MSR between guest value and host value on entry/exit. so, KVM has to do the switching with wrmsr/rdmsr instructions. A new feature, "load IA32_SPEC_CTRL" VMX control (refer to Chapter 15 in ISE spec[*]), can help but it isn't supported on SPR. [*]: https://cdrdv2.intel.com/v1/dl/getContent/671368 > > https://lore.kernel.org/kvm/20240410143446.797262-1-chao.gao@xxxxxxxxx/ > >Adding Chao for their opinion.
