Re: [PATCH v5 18/18] kvm: arm64: Allow tuning the physical address size for VM

From: Peter Maydell
Date: Tue Sep 18 2018 - 13:15:28 EST

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 ?

-- PMM