On Thu, Sep 12, 2024 at 10:29 PM Chao Gao <chao.gao@xxxxxxxxx> wrote: > > 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. I believe that is the case here. The guest's effective value is 1025. If the guest knew about BHI_DIS_S, it would actually set it to 1025, but older guests set it to 1. The IA32_SPEC_CTRL mask and shadow fields should be perfect for this. > "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. >
