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    SubjectRe: [PATCH v5 18/18] kvm: arm64: Allow tuning the physical address size for VM
    On 18 September 2018 at 17:27, Suzuki K Poulose <Suzuki.Poulose@arm.com> 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

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