On Mon, Sep 07, 2020 at 09:21:02 +0800, Zhenyu Zheng wrote: > Thanks alot for the reply, > > This sounds like a valid use case (not sure it is that useful), but we > > need to be careful. But we should make sure implementing this does not > > break anything. This means, we need to do different things depending on > > the type of the CPU definition we are asked to compare. Guest CPU > > definitions should keep the old behaviour (return IDENTICAL) and host > > CPU definitions can be compared to the host CPU. But when doing so, > > don't get too influenced by x86 code, which I believe is way too > > complicated for ARM. Specifically you don't need to create armCompute > > and mess with guestData and other stuff there as all you want to do is > > compare the two CPU definitions. In x86 the same function is used for > > several things, but that's not the case for ARM. > > > for this, what I have in mind now is that we can check > `VIR_CPU_MODE_HOST_PASSTHROUGH` > and if that is the case, we compare CPU vendors and models to allow only > identical definitions to pass, like the implementation of > https://gitlab.com/libvirt/libvirt/-/blob/master/src/cpu/cpu_ppc64.c#L505 , This doesn't sound like a good idea. PPC is quite special in the way it uses host-model and host-passthrough. But anyway, it's actually easier to just return IDENTICAL for host-passthrough CPUs (and all other guest CPUs) than copying the host CPU from capabilities and comparing it to itself. > (this is because when a VM is in host-passthrough mode, its' CPU xml > reflects the original host CPU > definition, and we actually compare the source host and destination host > CPU definitions, The CPU definition of a domain with host-passthrough remains the same. That is, it doesn't reflect the original host CPU definition, it's still just host-passthrough. The only way to compare source and destinations host CPUs is by taking the host CPU def from one host and passing it to cpu-compare called on the other host. Jirka
