On Thu, 2023-03-02 at 13:36 +0800, Gao, Chao wrote: > On Wed, Mar 01, 2023 at 11:54:38PM +1300, Kai Huang wrote: > > Make setup_vmcs_config() preemption disabled so it always performs on > > the same local cpu. > > > > During module loading time, KVM intends to call setup_vmcs_config() to > > set up the global VMCS configurations on _one_ cpu in hardware_setup(), > > and then calls setup_vmcs_config() on all other online cpus via sending > > *all other* is misleading. The compatibility check is actually done on > *all* online cpus. > > for_each_online_cpu(cpu) { > smp_call_function_single(cpu, kvm_x86_check_cpu_compat, &r, 1); > if (r < 0) > goto out_unwind_ops; > } > > Given this, it probably is ok to not disable preemption because all CPUs > are guaranteed to be compatible later in the flow in terms of VMCS > capabilities. But we don't want to have such a subtle dependency. > > Do you see any real problem with preemption enabled? No. Just thought it is the right thing to do. > > > IPI to perform VMX compatibility check. Further more, KVM has CPU > > hotplug callback to call setup_vmcs_config() to do compatibility check > > on the "new-online" cpu to make sure it is compatible too. > > > > setup_vmcs_config() is supposed to be done on the same cpu. This is > > true in the compatibility check code path as setup_vmcs_config() is > > called either via IPI or in per-cpu CPU hotplug thread. However, the > > first call from hardware_setup() isn't as it is called when preemption > > is enabled. > > > > Change the existing setup_vmcs_config() to __setup_vmcs_config() and > > call the latter directly in the compatibility check code path. Change > > setup_vmcs_config() to call __setup_vmcs_config() with preemption > > disabled so __setup_vmcs_config() is always done on the same cpu. > > Maybe you can simply disable preemption in hardware_setup() although I > don't have a strong preference. > > nested_vmx_setup_ctls_msrs() also reads some MSRs and sets up part of > vmcs_conf, should it be called on the same CPU as setup_vmcs_config()? Yes I think so. I missed this :) Not sure whether there are other similar places too even outside of hardware_setup(). But compatibility check only checks things calculated via setup_vmcs_config() and nested_vmx_setup_ctls_msrs(), so I think it's fair to only put hardware_setup() inside preemption disabled.