On 11/02/20 23:38, Sean Christopherson wrote: > On Tue, Feb 11, 2020 at 07:04:39PM +0100, Paolo Bonzini wrote: >> The microcode version should be set just once, since it is essentially >> a CPU feature; so do it on vCPU creation rather than reset. > I wouldn't call it a CPU feature, CPU features generally can't be > arbitrarily changed while running. That was true of CPUID bits too until microcode started adding and removing them, but I see your point. :) What I was trying to convey as "CPU feature" is that KVM will not change it arbitrarily when running; it can only change as a result of userspace actions, KVM_SET_MSRS in this case. But yes, I will improve the text based on your version: --- Do not initialize the microcode version at RESET or INIT, only on vCPU creation. Microcode updates are not lost during INIT, and exact behavior across a warm RESET is not specified by the architecture. Since we do not support a microcode update directly from the hypervisor, but only as a result of userspace setting the microcode version MSR, it's simpler for userspace if we do nothing in KVM and let userspace emulate behavior for RESET as it sees fit. --- Thanks, Paolo > I'd prefer to have a changelog that > at least somewhat ties the change to hardware behavior. > > Do not initialize the microcode version at RESET or INIT. Microcode > updates are not lost during INIT, and exact behavior across a warm RESET > is microarchitectural, i.e. defer to userspace to emulate behavior for > RESET as it sees fit. > > For the code:
