Re: [libvirt] [RFC] 5-level paging Support

On Wed, Dec 13, 2017 at 08:43:38PM +0800, Feng, Shaohe wrote: > Hi all, > > Now both qemu and kvm support 5-level paging. > > We can start qemu with a "cpu,+la57" to set 57-bit vitrual address space. > > So VM can be aware that it need to enable 5-level paging. > > > We can also set another "cpu,phys-bits=52" to set the VM physical address > space. > > Actually, VM can still turn on 5 level paging even without "phys-bits=52", > yet this means > > the guest physical address width are limited, meaning less practical > benefits. > > > In to support 5-level paging, I suggest to add two attribute for the domain > cpu element in libvirt. > > <cpu la57='yes', phys-bits='52'\> This doesn't make sense - 'la57' is just a CPU feature like any other CPU feature reported in /proc/cpuinfo. IOW, we should just have <cpu> .... <feature name="la57" policy="require"/> </cpu> > If la57='yes', libvirt will probe the host capability, and will throw error > if hardware does support la57. I'm not seeing why we need to special case la57 handling, when we can just use standard CPUID checks we already have > If la57='yes', even phys-bits less than 52, libvirt will also pass it to > qemu though less practical benefits. I would suggest separating la57 from phys-bits, as they're actually completely independant features. They merely happen to be commonly used at the same time.

Regards, Daniel