On 26/02/2018 13:16, Paolo Bonzini wrote: > On 26/02/2018 13:15, Borislav Petkov wrote: >> On Mon, Feb 26, 2018 at 12:54:52PM +0100, Paolo Bonzini wrote: >>> I don't understand how one thing follows from the other. How are writes >>> to 0x8B related to having a virtualized microcode loaded (which is a >>> concept that actually makes no sense at all)? >> >> I'm questioning the whole idea. 0x8b is the MSR which gives you the >> microcode revision. Most CPUs don't even allow writing to it, AFAICT. >> (SDM says "may prevent writing" on VM transitions.) >> >> So how is that host-initiated write to 0x8b is even going to work, in >> reality? kvm module writes the microcode version in there? How does the >> admin work around that? > > In this context, "host-initiated" write means written by KVM userspace > with ioctl(KVM_SET_MSR). It generally happens only on VM startup, reset > or live migration. To be clear, the target of the write is still the vCPU's emulated MSR. Paolo
