Re: [PATCH RESEND] accel/kvm: Specify default IPA size for arm64

[Peter Maydell]

On Thu, 20 Jul 2023 at 08:29, Akihiko Odaki <akihiko.odaki@daynix.com> wrote:
>
> libvirt uses "none" machine type to test KVM availability. Before this
> change, QEMU used to pass 0 as machine type when calling KVM_CREATE_VM.
>
> The kernel documentation says:
> > 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 size, 40bits (for backward compatibility)
> >   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.
> >
> > Creation of the VM will fail if the requested IPA size (whether it is
> > implicit or explicit) is unsupported on the host.
> https://docs.kernel.org/virt/kvm/api.html#kvm-create-vm
>
> So if Host_IPA_Limit < 40, such KVM_CREATE_VM will fail, and libvirt
> incorrectly thinks KVM is not available. This actually happened on M2
> MacBook Air.
>
> Fix this by specifying 32 for IPA_Bits as any arm64 system should
> support the value according to the documentation.
>
> Signed-off-by: Akihiko Odaki <akihiko.odaki@daynix.com>
> ---
> Resending due to inactivity.
>
>  accel/kvm/kvm-all.c | 4 ++++
>  1 file changed, 4 insertions(+)
>
> diff --git a/accel/kvm/kvm-all.c b/accel/kvm/kvm-all.c
> index 373d876c05..3bd362e346 100644
> --- a/accel/kvm/kvm-all.c
> +++ b/accel/kvm/kvm-all.c
> @@ -2458,7 +2458,11 @@ static int kvm_init(MachineState *ms)
>      KVMState *s;
>      const KVMCapabilityInfo *missing_cap;
>      int ret;
> +#ifdef TARGET_AARCH64
> +    int type = 32;
> +#else
>      int type = 0;
> +#endif
>      uint64_t dirty_log_manual_caps;
>
>      qemu_mutex_init(&kml_slots_lock);
> --
> 2.41.0

I think this was discussed in review the last time around on
this patch (which is why it wasn't applied), but I don't think
we should do this with a target ifdef in a common file. We should
have some kind of target hook for targets to specify what they need
to do here.

thanks
-- PMM