On 1 September 2012 10:11, Avi Kivity <avi@xxxxxxxxxx> wrote: > Other x86 state: > Control registers: ok. Should userspace be careful to set registers > in legal ways only? i.e. cannot set cr3[0:11] if cr4.pae=0, or vice > versa, so need three writes? The principle I'm hoping we can hold to for ARM is that the kernel only exposes state in such a way that it's always possible for userspace to migrate it with a simple "read everything, send to destination, write everything", ie without needing to know anything of the semantics of any of these registers. This means that registers which have access controls (eg "can't write this unless you wrote to that other one first") should not enforce those checks for userspace get/set. More significantly, it means that registers with odd behaviour, like "write 1 to clear" or "register A selects which of an array of underlying registers is exposed in register B" are not directly exposed at all. Instead the kernel provides some other (ersatz) register indexes which let userspace do plain get/set on the underlying state. The idea is that then migration depends only on whether the destination kernel supports all the registers the source kernel does, and we avoid extra dependencies on the source and destination qemu. (Most of the state being transferred is of no interest to userspace at all.) [It also makes write-multiple easier to use.] -- PMM -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
