On 18 September 2018 at 17:27, Suzuki K Poulose <Suzuki.Poulose@xxxxxxx> wrote: > --- > > "On arm64, the physical address size for a VM (IPA Size limit) is limited > to 40bits by default. The limit can be configured if the host supports the > extension KVM_CAP_ARM_VM_IPA_SIZE. When supported, use > KVM_VM_TYPE_ARM_IPA_SIZE(IPA_Bits) to set the size in the machine type > identifier, where IPA_Bits is the maximum width of any physical > address used by the VM. The IPA_Bits is encoded in bits[7-0] of the > machine type identifier. > > e.g, to configure a guest to use 48bit physical address size : > > vm_fd = ioctl(dev_fd, KVM_CREATE_VM, KVM_VM_TYPE_ARM_IPA_SIZE(48)); > > The requested size (IPA_Bits) must be : > 0 - Implies default 40bits (for backward compatibility) > > or > > N - Implies N bits, where N is a positive integer such that 32 <= N <= > Host_IPA_Limit > > Host_IPA_Limit is the maximum possible value for IPA_Bits on the host and > is dependent on the CPU capability and the kernel configuration. The limit > can > be retrieved using KVM_CAP_ARM_VM_IPA_SIZE of the KVM_CHECK_EXTENSION > ioctl() at run-time. > > Please note that configuring the IPA size does not affect the capability > exposed by the guest CPUs in ID_AA64MMFR0_EL1[PARange]. It only affects > the guest to host physical address (stage2) translations setup by the host. > " Thanks, this is much clearer. The only bit I'm not sure about is that last paragraph -- if I ask for a VM with a 48 bit address space why don't we tell the guest that that's what it has ? thanks -- PMM
